It has been long ago that I posted something extensive on wild horses here on my blog, which is why I decided to do a summary of all my past posts on the biology of western Eurasian wild horses. I have been digging through a lot of literature since 2013 and am also going to add some new information. I hope you find useful.
Definition & population genetics
At first I want to define what kind of wild horses I have been focusing on. As long as we regard Przewalski’s horses as members of the same species as domestic horses, Equus ferus, which is what I do, this species is composed of two major clades: one leading to the Przewalski’s horse, Equus ferus przewalskii, and one leading to domestic horses including their wild progenitors, Equus ferus ferus (=Equus ferus caballus). Genetics suggest a separation of both lineages between 160.000  and 38-72.000 years . On my blog, I have been focusing on these extinct wild horses that spread from Iberia to the Eurasian steppe and were domesticated about 6000 years ago.
This is where it gets complicated now, as there are several more or less open questions: How long did true predomestic ferus-type wild horses survive in Europe? Are the historic accounts referring to true predomestic wild horses, hybrids with feral horses or feral populations and wild horses were already extinct by that time after all? Where was the line between the geographical range of the ferus-subspecies and przewalskii-subspecies? (the later question is hardly addressed in the literature).
Not only are these questions hard to answer, there are also some uncertainties on how to refer to these populations. A very popular term is “Tarpan” (in the German-speaking non-scientific literature, it is also very common to distinguish between a “forest tarpan” and a “steppe tarpan” – a distinction that mostly came about by “copy+paste” and is baseless ). This term is problematic, as there is no certainty on what kind of animals it originally described. It has become a trend to either condemn the term, or to strictly define it as the term for predomestic, ferus-type wild horses, no matter what it originally described. You have to choose it based on your taste. But if you choose for the latter option, you have to be aware of the fact that “tarpan” has no tradition of being colloquially used for the native European wild horses. It never was a colloquial term outside of the steppes in the way “deer”, “wolf” or “aurochs” was. It has been introduced in scientific literature in 1763 , so it is just convention-based. In my 2013 post What do we mean by “Tarpan”? I chose this option and continued to used this term for another few blogposts. However, nowadays I avoid using the word “tarpan” because of all the confusions and also the connection to baseless concepts like “forest vs steppe tarpan” or the Konik-story and others (more on that in an upcoming post). Then I increasingly referred to the ferus-type wild horses as the European wild horse(s), or the Western wild horse(s) (in contrast to the Eastern wild horse, the Przewalski’s horse).
Today I still like to use the latter two terms, usually in plural, as we do not know how many local types there were. Ancient DNA samples indicate two separated predomestic populations on western Eurasia for the Holocene, one on the Iberian peninsular and one extending to the Eurasian steppe . That does not mean that the Pyrenees were the mark were we can find phenotypic differences between the horse populations. It means that at least for some time it was a reproductive barrier. It is possible that there were indeed phenotypic differences between the wild horses dwelling the European mixed forest regions and those in the steppes, as both are two very different biomes. For that, we have to look at the evidence. Furthermore, any differences between wild horses of the European mixed forest biome* and the western steppe biome must have been more or less continuous, because so were the populations.
*I do not claim this biome was densely forested all over. This is not likely by current evidence, but neither is a “European savannah”. Rather, I assume a mosaic of open, semi-open and closed habitat for this biome.
Coat colour genetics
Historic references are a problematic source as it is not clear what kind of free-roaming horses they actually referred to. Ancient DNA, however, can at least provide a clue on the colour phenotypes present in predomestic European horse populations. A number of studies have been conducted in the past, identifying colour alleles in prehistoric European wild horses.
We mainly have to deal with four colour phenotypes that have been proven for predomestic horses, that I briefly introduce now:
Bay: This phenotype is caused by the expression of the dominant allele A and recessive non-dun1. Wild markings like the dorsal stripe are still visible (non-dun2, however, is a domestic mutation that does not show the wildtype markings).
Bay dun: This colour is produced by the bay base colour plus the dominant Dun allele. Wildtype markings are more or less clearly visible (dorsal stripe, shoulder stripes, leg stripes)
Black: Black is recessive under bay and caused by the a allele.
Black dun: Black dun is caused by a black base colour plus the dun allele. It is also referred to as “grullo”, “blue dun”, “mouse dun” and other terms and present in various shades. Wild markings are more or less clearly visible.
(There is also evidence for the so-called “leopard spotted” wild horses , but I leave this colour variant aside for now)
Bay dun is also found in other wild equines such as the onager, kiang and Przewalski’s horse, and therefore was ancestral for the ferus-lineage. It is also dominant over all the other colour phenotypes. Bay was the only Agouti allele in the Pleistocene, as the black variant is a mutation that occurred on the Iberian peninsular during the Holocene, where it started to outnumber bay and spread eastwards , indicating some selective advantage. What is interesting is that the non-dun1 mutation existed 42.700 years ago at least, and side by side with Dun. Therefore in the Pleistocene, wild horses of the same population either showed a bay dun or bay phenotype (the former might have been more common as it is dominant), while for the Holocene of Europe we have four possible phenotypes – bay dun, bay, black dun and black – as there was no regional or chronological gradient for the dun factor found. In sample of Pruvost et al. for Holocene wild horses of Europe and the steppe, the bay allele was found more often, while in Ludwig et al., the black allele was found to be prevalent on the Iberian peninsular (75%).
This of course provokes two very interesting questions: what was the selective advantage of the black allele (Ludwig et al. speculate that this colour variant was of advantage in an increasingly forested habitat in the Holocene, I consider that plausible), and why where ferus-type wild horses one of the very few large vertebrate species that apparently was heterogeneous in colour? One explanation for the latter question is that maybe we are looking at a mutation-and-selection process that was under its way there and perhaps, if the populations had not been driven to extinction by man, would have become homogeneous in colour after a couple of further millennia of evolution.
Back in 2013, I illustrated the four respectively five colour phenotypes that occurred in the wild ferus subspecies of the Holocene:
The drawing also shows countershading in combination with the white muzzle (“mealy mouth”). It is, based on parsimony, ancestral for the ferus-lineage as well, but as far as I know the genetic background for it has not been resolved yet and also not been tested in aDNA samples, which is why its presence on my scheme is speculative. Countershading can be visible on a black dun phenotype, but never is the white muzzle. This drawing that I recently did shows a bay dun wild horse without countershading and white muzzle (it is actually part of a larger drawing that I am going to present in the next few days):
As an interesting side fact, the sorrel mutation of domestic horses apparently found its way into wild horse populations via introgression (Pruvost et al.).
Coat colour is of course only one aspect of the animals’ life appearance. For the morphology of ferus-type wild horses, we have to look at other sources. If you wonder why I illustrated my wild horses with a falling mane, I am going to come back to that later on.
Due to the large osteological similarity between wild and domestic horses, it is very difficult to distinguish between both types in the subfossil record. Usually, when the bones are associated with human material, they get assigned to the domestic type unless they show signs of hunting, but the only way to distinguish them safely is on genetic level (I do not know how the studies for wild horse coat colour genes detected wild horse material, but I hope it was one genetic level). In the literature you barely find any remarks on the morphology of Holocene ferus-type wild horses based on osteologic material, and you also never see mounted skeletons of such horses in museums. While it is probably true that the mixed-forest biome is not the primary habitat of horses (I am not claiming that this biome naturally consisted only of closed forests, not at all), Holocene European wild horses could not have been that dramatically rare that one does not even have enough material for a couple of mounted skeletons. I suspect the true reason is that most Holocene subfossil equine material not directly associated with humans have been assigned “Equus sp.” and have been getting dusty in collections since then. I suggest testing some of this material and collect a number of genuine subfossil wild horse specimen, and I am confident that there is enough material for at least a couple of mounted specimen that could give us a clue on the actual morphology of Holocene European wild horses.
The only remarks describing the morphology of alleged European wild horse material were referring to comparisons with Exmoor and Konik bones – not surprisingly, the authors reported large similarity [8,9]. I say “not surprisingly” because the morphology of Holocene ferus-type wild horses was probably comparable to a robust pony type as this body type is also typical of Pleistocene remains and the Przewalski’s horse. However, one has to be careful of self-fulfilling prophecies – that is, if there were indeed ferus-type wild horse remains with a warmblood-like morphology, they might automatically become assigned to warmblood-type domestic horses because our conception of wild horses expects us only to find pony-like remains. What I am saying is that it cannot be bad to strengthen this conception on empirical grounds by examining the subfossil horse record more thoroughly, also to get an idea of regional variation (f.e. it could be possible that wild horses got larger and more long-legged towards the Eurasian steppe as the habitat becomes more open).
Historic accounts of free-roaming horses in Europe and the steppes
As genetic information so far only told us about the coat colour of ferus-type wild horses in Europe, and osteologic material has not been examined thoroughly enough yet to draw solid conclusions on morphology, size and regional variation, we also have to rely on human sources of information: that is, contemporaneous art and written sources. I know of no historic artworks showing supposed European wild horses (except for a “Tarpan” by C. H. Smith from the 19th century, that is way too generic). I do not use Pleistocene cave paintings as source material as the climate and therefore the biome and consequently the wild horse type of the Pleistocene in Europe were not comparable to those of the Holocene.
Written historic sources are problematic for the reasons I outlined above in the beginning of the post. It is not known when original, un-hybridized ferus-type wild horses died out. It could be that they died out in prehistoric times already, and that all historic accounts refer to feral domestic horses that exhibited more or less primitive traits. It is also possible that these populations were hybrids of wild and abandoned/escaped domestic horses. For the texts referring to free-roaming horses of the Russian steppe it becomes increasingly complicated because it is not known how far westwards the original range of the Przewalski’s horse extended, and if those wild horses described in the texts were actually either Przewalski’s horses or hybrids between those and feral horses, and not ferus-type wild horses.
One aspect why some authors discard all historic accounts of free-roaming horses a priori is that they seem to mention falling manes. A falling mane has long been viewed as a domestic trait, as all extant wild equines have a short erect mane. However, this conception is out-dated. Frozen Pleistocene mummies assigned to Equus lambei, a taxon suggested to be conspecific with Equus ferus , have a falling mane, showing that this trait arose well before domestication. It has been suggested that a falling mane and a bushy tail evolved as a protection from precipitation , and indeed all extant equines with an erect mane live in arid environments with low precipitation. So it is not unlikely that the Holocene wild horses of Europe had a falling mane. It is also most likely that the ancestral population of the domestic horse and the last common ancestor with E. lambei had a falling mane, otherwise this trait evolved twice.
Therefore, a reported falling mane is not an indication of feral horses. There are also authors that read “short mane” (a term used in some of these accounts) as an evidence for erect manes in Holocene wild horses of Europe, but “short” does not automatically mean “erect”, it can also describe a short but falling mane. And most commonly, the sources write of “short and frizzy” manes, indicating the manes were actually falling or at best semi-erect (what also occurs in some domestic horses).
It is not impossible that genuine, un-hybridized wild horses survived in Europe and the steppes until the 19th century. As the subfossil record of predomestic equines is understudied, there is no hint that these populations disappeared before historic times. It is possible, but the contrary is just as likely. However, it is questionable how pure these populations were. Wild horses were reported to steal domestic mares from farms, which is one of the reasons why they were persecuted by man, and must have led to occasional domestic introgression into wild populations. But the influence of escaped or abandoned cavalry horses that ran wild must have been more dramatic, something that happened for centuries. Wild and domestic forms usually tend to hybridize wherever they share the habitat, which is also apparent in canines and pigs. Furthermore, the notion by Pruvost et al. that the sorrel mutation found its way into predomestic populations shows that hybridization did took place.
I believe the truth is that we cannot know what the true identity of the historic free-roaming horse populations of Europe and the western Eurasian steppe was. A number of the historic accounts describe horses with morphological and behavioural traits expected or suspected for wild horses: a small, sturdy body, short mane, large head, wildtype colour (as explained above, there are four to five wildtype colour variants for ferus-type wild horses), shy behaviour that is either untameable or tameable to a certain degree after some time (other wild equines also can be tamed more or less). Due to the morphology and behaviour that is described, I think it is quite plausible that at least some of these populations represented actual wild horses, with a varying degree of influence from domestic horses. Nevertheless, suspected wildtype traits described in those populations would also be found if they descend from rather primitive landraces. And once again, we have to beware of the danger of self-fulfilling prophecies or circular reasoning.
In my 2014 post Western wild horses: What does the evidence actually say? I presented all sources of written accounts on putative ferus-type wild horses that I was able to find. I noted whether I got it from a primary source (therefore the actual text itself), a secondary source (a text that describes what is written in the actual source) or a tertiary source, in order to prevent the danger of Chinese whispers. Another inevitable problem is the language issue, especially with sources from the antiquity. Furthermore, there can be confusion over equine colour terminology because the terms changed over time. For example, “dun” used to refer to bay dun exclusively, and it is still used this way in the UK today, while in the strict modern terminology backed up by a genetic basis “dun” refers to a modification of pigmentation that can act upon variable base colours. Thus notions of an “Isabella” or “tan” colour from the 18th century must not be the same genetic colour variant as in modern terminology. The interpretation of “mouse-colour”, which is mentioned rather commonly in those historic texts, is subjective as well. It is usually interpreted as referring to black dun horses, which is plausible, but it is also possible that individual authors had other colours in mind.
I copy the summary of historical sources from 2014 into this post, also giving a personal interpretation. A P indicates a primary source, a S a secondary source, and a T a tertiary source.
Herodot, 5th century before Christ: lightly coloured (leukos) wild horses in the Ukraine. (S)  I am not a classical philologist, but I read that “leukos” can either be translated with “white” or “light-coloured”. A white colour would indicate a domestic origin of the horses described, light-coloured can indicate dun (and therefore wildtype colour) among other colours.
St. Isidore, 600: Iberian wild horses: colour like a donkey, ash-coloured. (P)  He is probably referring to black dun horses, as this colour scheme is rather similar to that of a donkey.
Albertus Magnus, 13th century: greyish-coloured wild horses with a dorsal stripe in Germany (S).  Very likely black dun as well.
F. Chr. Dahlmann, 1840: Large numbers of wild horses that were hunted lived in Denmark of the 12th century (T).  No information on the looks of the horses is given.
H. Röslin, 1593: Wild horses were still living at Vogesen, Elsass-Lothringen. They were faster and wilder than deer and difficult to catch. Once caught, they got tame after some time (S).  No information on the looks are given, could be either feral or wild horses.
Balthasar Haquet, 18th century: Wild horses at Zamosc: small, blackish brown, large and thick heads. Mane and tail comparably short, stallions had a beard. Were used in fight shows with predators and showed great bravery (S).  The described morphology suits the prevalent conception of wild horses, and “blackish brown” colour could either refer to black dun or other (in this case domestic) colour variants.
Eugeniusz Rozdzynski, 1721: Wild horses at Zamosc: tan and isabelline in colour (T).  “Tan” can indicate a lot of colour variants (bay, seal brown, dark expressions of black dun, chestnut), and isabelline could either indicate isabelline in the modern sense or any light colour, in which case dun variants would be among them. Thus, the description could refer to the wildtype colour variants, but also others.
Rytchkof, 1762: colour dun or bluish, other shades exceptional (S).  “Dun or bluish” most likely refers to both bay dun or black dun, or various expressions of black dun.
Samuel Gottlieb Gmelin, 1768: Wild horses at Woronesh, Russia. The largest of those wild horses barely reached the size of the smallest Russian domestic horses. The ears were very pinned and the size of domestic horses or sometimes longer and hanging down. The eyes were fierily. The mane was short and frizzy, the tail shorter than in domestic horses. The were typically mouse-coloured, but white or grey horses were also reported. The belly was ashy at the base, the legs were black from the knees downwards. The hair was long and dense during the winter and felt more like fur than horse hair. (P, S).  It seems plausible that the author was indeed referring to black dun, white and grey. The morphology fits what is expected for wild horses.
Berenger, 1771: Ural. middling size, roundish and short, big heads, ewe-necked and of a bluish grey colour (S).  Once again plausible wild horse morphology. Bluish grey colour could either refer to expressions of black dun or grey.
Peter Pallas, 1771: Free-roaming horses at eastern Prussia and western Siberia. Considered them feral domestic horses. Resembled Russian farm horses, but they had thicker heads, pinned ears, short and frizzy manes and shorter tails. They were of a greyish brown colour and had lighter coloured legs, brown and greys would appear. The colour of the head was white/light on the snout and black towards mouth. Black horses were rare, and there were no piebald ones. They lived in herds of 20 individuals. (S) [8,11] “Greyish brown colour” probably indicates black dun, but “lighter coloured legs” would not fit this colour variant, as it includes dark coloured legs. However, as he also mentions other colour variants, it is not clear which of those had lightly coloured legs. Both “brown” (might refer to bay) and black colours are possible for ferus-type wild horses.
Kajetan Kozmian, 1783: Wild horses at Zamosc: small, strong limbs, enormous strength and uniform dark mouse colour (S).  The described morphology is plausible for wild horses, dark mouse colour probably refers to black dun and the notion that the colour was uniform might indicate that the herds he saw were not admixed.
C. H. Smith, 1841: (Probable) Wild horses in the Russian steppe: “coupled with different proportions and position of the ears, an arched or plane forhead, a straight or curved nose, a difference of colour in the eyes, of the skin, of the hoofs, the constancy of their liveries, of their marks, in a streak along the back and bars on the limbs, of dappled croups and shoulders, or of dark uniform colours, dense or thin manes and tails, although traits now mixed,” […] “All seem to refer to a sturdy form of mountain-forest ponies, still found in the province of Cordova, in the Pyrenees, the Vogesian range, the Camargue, the Ardennes, Great Britain, and in the Scandinavian highlands: all remarkable for an intelligent but malicious character, broad forheads, strong lower jaws, heavy manes, great forelocks, long bushy tails, robust bodies, and strong limbs; with a livery in general pale dun, yellowish brown and a streak along the spine and cross bars on the limbs, or the limbs entirely black, as well as all the long hair and mostly having a tendency to ashy and gray, often dappled on the quarter and shoulders”. […]“These horses are evidently again referred to by Andr. Schneebergius, who states, that “there were wild horses in the preserves of the prince of Prussia, resembling the domestic, but mouse-coloured, with a dark streak on the spine, and the mane and tail dark;” […] “Real Tarpans are not larger than ordinary mules, their colour variably tan, Isabella or mouse, being all shades of the same livery, and only varying in depth by the growth or decrease of a whitish surcoat, longer than the hair, increasing from midsummer and shedding in May: during the cold season it is long, heavy, and soft, lying so close as to feel like a bear’s fur, and then is entirely grizzled; in a summer much falls away, leaving only a certain quantity on the back and loins: the head is small, the forehead greatly arched, the ears far back, either long or short, the eyes small and malignant, the chin and muzzle beset with bristles, the neck rather thin, crested with a thick rugged mane, which, like the tail, is black, also the pasterns, which are long: the hoofs are narrow, high and rather pointed; the tail, descending only to the hocks, is furnished with coarse and rather curly or wavy hairs close up to the crupper; the croup as high as the withers: the voice of the Tarpan is loud, and shriller than that of a domestic horse; and their action, standing, and general appearance, resembles somewhat that of vicious mules.” (P)  For Smith’s full text, go here.
C. R. Darwin, 1868: “It seems that not very long ago a wild breed of dun coloured horses with a spinal stripe was preserved in the royal parks in Prussia. I hear from Hungary that the inhabitants of that country look at the duns with a spinal stripe as the original stock, and so it is in Norway”. (P)  He is probably referring to the population at Zamosc.
Heptner, 1989: Last living Tarpan (Dubrowka Tarpan), died 1918. It was 140-145 cm tall, had a large head, small ears, short neck, mouse-coloured coat, broad dorsal stripe, faint shoulder stripes, black mane, tail and lower legs, semi-erect mane, broad and arched front head and a straight head profile. (S) 
I have not commented the remarks of Smith and Heptner yet because I treat the problematic of the steppe horses as a separate issue. At first, I want to come back to the horses of Europe western to the steppes. The colour that is mentioned most frequently is probably black dun. While in genetic samples bay was the prevalent base colour, there was a tendency of the black allele becoming increasingly common, as mentioned above. If this trend continued to historical times, the black allele might have outnumbered the bay allele. So the prevalence of black dun horses in written accounts is plausible for wild horses. For Iberia, St. Isodore mentioned probably black dun horses exclusively (“donkey coloured”), while Smith describes a southwestern European horse type that might have displayed both bay dun and black dun (“pale dun and yellowish brown” vs. “ashy to grey”). Therefore, the described coat colours often match with what is expected from genetic information. But it cannot be denied that many of the sources also mention colour variants not proven for wild horses, i.e. most likely domestic colours, such as white/grey and perhaps the notion of “isabelline” and “tan” horses (however, both words can also indicate bay and dun horses, which would be wildtype colour variants). It is of course possible that all those free-roaming horse populations were feral, primitive rural horse type and hence the wild horse-like behavioural and optical traits. But it is, in my opinion, just as likely that they do represent wild horse populations that mixed with abandoned/escaped domestic horses to a varying extent. It is a tricky situation: it would be as if it was not known when genuine aurochs eventually disappeared, but there would be historic accounts of very aurochs-like bovines ranging freely, looking a lot like aurochs but some individuals having diverging colour or horn variants. True aurochs, feral primitive cattle, or hybrids?
Only a genetic test of a representative sample of individuals from those populations could provide clarity.
For the western steppes, it becomes even more complicated, as it is not clear how far westwards the Przewalski’s horse originally ranged. Artworks from the 4th and 5th century before Christ resemble Przewalski’s horses in having an erect mane and a short-haired tail base , which makes it likely to me that this subspecies extended at least until the south-western edge of the steppes. The records of putative wild horses from the western steppes could easily also refer to Przewalski’s horses or hybrids of Przewalski’s horses with feral horses. Especially the notion of the “Tarpan” resembling “vicious mules” is reminiscent of the Przewalski’s horse with its standing mane and large, donkey-like head. It was probably not coincidental that this equine was initially described as Asinus przewalskii.
The records of three particular individuals from these free-roaming steppe horses are problematic too. One of them, the “Krim Tarpan”, was suspected to be a feral horse already back in the 19th century. The purportedly last individual of these populations, the “Dubrowka Tarpan”, fits the conception of a wild horse except for its comparably large size (see above), at least according to Heptner 1989. It is possible that it either a) was indeed a wild horse (wild horses are usually thought to be smaller, but as I wrote above there is no comprehensive subfossil record of articulated skeletons ferus-type wild horses from which we could get an idea of their size, especially not from the steppe, so we cannot rule out that there were 145cm tall wild horses) b) a hybrid between a ferus-type wild horse and a feral horse, c) a hybrid between a Przewalski’s horse and a feral horse, d) a feral horse with some primitive traits. The most famous of those “tarpan” individuals is the “Cherson tarpan”, which was caught as a foal in the Ukraine and exhibited in the Moscow Zoo. It is famous as it is the only “tarpan” that was photographed. It displayed aggressive behaviour and was castrated, but otherwise showed none of the suspected wildtype traits clearly. The mane was opulent and long, the legs were long too, and the colour could have been either bay, seal brown or a very dark shade of black dun to my eyes. With a size of 133cm at the withers, it fitted the expected height for wild horses, but so do many rural horses. It could simply be a feral horse, based on its looks I see nothing compelling that would qualify it as a genuine, un-hybridized wild horse.
As you see, there are a lot of open questions, room for interpretation and subjectivity on this subject. Some of these can only be solved by conducting research (such as identifying complete osteologic material of genetically proven predomestic horses, to get an idea of their actual morphology), others probably have to remain unanswered (such as the status of the historic free-roaming horse populations, as no specimen are preserved).
The true nature of Holocene ferus-type wild horses is of course relevant for ecologic restoration in order to chose horse breeds that serve as the most authentic proxy. For a number of breeds there are background stories purported by their advocates as the most authentic and most “wild” European horses today, which are mostly based on Chinese whispers, arbitrary interpretations and fabrications. The ferus-type wild horse is as extinct as the aurochs. I already wrote several posts on this subject. When I can find the time, I am going to do a similar summary on this issue.
For the posts that I wrote in the past and used as a basis for this summary, go here:
- What theTarpan looked like (2013)
 Ryder et al.: A massively parallel sequencing approach uncovers ancient origins and high genetic variability of endangered Przewalski’s horses. 2011.
 Orlando et al.: Recalibrating Equus evolution using the genome sequence of an early Middle Pleistocene horse. 2013.
 Tadeusz Jezierski, Zbigniew Jaworski: Das Polnische Konik. 2008.
 Cieslak et al.: Origin and history of mitochondrial DNA linages in domestic horses. 2011.
 Pruvost et al.: Genotypes of predomestic horses match phenotypes painted in paleolithic works of cave art. 2011
 Ludwig et al. 2009: Coat color variation at the beginning of horse domestication
 Imslandet al.: Regulatory mutations in TBX3disrupt asymmetric hair pigmentation that underlies Dun camouflage colour inhorses. 2015.
 Tadeusz Jezierski, Zbigniew Jaworski: Das Polnische Konik. 2008. Polish Academy of Sciences
 Baker, Sue: Exmoor Ponies: Survival of the Fittest – A natural history. 2008.
 Kirkpatrik, Jay F.; Fazio, Patricia M.: Wild horses as Native North American Wildlife.2005
 Charles Hamilton Smith, 1841: The Natural History of horses, with Memoir of Gesner
 Hardy Oelke: Wild horses then and now. Kierdorf-Verlag.
 Cis van Vuure: On the origin of the Polish konik and its relation to Dutch nature management. 2014.