The European wild horse probably was of a similar build like the Przewalski’s horse, that means a 130-140 cm tall stocky body with a large and robust head. Numerous eyewitness accounts confirm that, as well as that it had a short but falling mane. But what about the exact colour? Genetic information tells us about the two basic colours that were present in European wild horse populations, but the phenotype of these horses is uncertain as long as we don’t know if the basic colour was diluted by dun or not.
To make more clear what I am about, let’s have a deeper look into the coat colour genetics of horses. Horses have two alleles on their Extension locus, which defines the basic colour: E, the wild type allele, resulting in the production both red and black pigmentation, and the domestic mutation e, which is recessive and produces totally red colours such as chestnut or sorrel. The amount of black pigmentation in wildtype coloured horses is controlled by the Agouti locus. The allele A restricts black to the legs, mane and tail and results in bay horses; the allele a on the other hand produces totally black horses. We know that both A and a are wild type alleles, they were present in undomesticated European horse populations . There is another variant called At, producing colours like seal brown. This allele is not mentioned by Pruvost et al., but it might indeed be another wildtype allele;
according to this website, it might even be present in donkeys as well (if
that’s the case, it’s a very old allele). Countershading, also called pangare, which produces the mealy mouth and light belly in bay and bay dun (but not black and black dun) horses, serves the so-called somatolysis and is present in a variety of animals of all sizes and habitat preferences, so it doesn't say much on whether we look at a forest-dwelling animal or open land animal.
Dun is the only dilution locus which is wildtype. Dun reduces the intensity of pigmentation; bay and seal brown horses with dun are sand-coloured (bay dun) while black horses with dun are grayish or tan-coloured (black dun), also called grullo. The so-called wild markings including the dark dorsal stripe and the leg striping are certain indication of the presence of dun since the former trait is barely visible and the latter invisible without dun. All living equines are dun-coloured (bay dun in particular), so it sounds logic that European wild horses were too. On the other hand, all living wild equines live in open habitats, while Europe likely had more closed, forest-based natural landscapes. Therefore the loss of dun resulting in a darker colour (a more efficient camouflage in forests) seems conclusive as well. And indeed bay/brown and black ponies like the Exmoor, Asturcon or Fell pony seem to fit closed or bushy environment much better than do bay dun and black dun Przewalskis or Koniks. Since there are no preserved skins of undomesticated native European horses, we have to ascertain that question using two sources of information: genetic information and historic accounts. Cave paintings could provide a clue as well, but they are not very reliable for two reasons: 1) they stem from the age of the last glacial, and therefore the horses depicted were most likely adapted to more steppe-like habitat than that of modern Europe, and 2) probably not all colours on the cave walls are an accurate representation of what was found within the population because the cave artists had to work with what was available.
|Black dun Konik: perfectly camouflaged in open, grassy landscapes|
|Dark bay Exmoor: perfectly camouflaged in more closed environments|
Genetic data is by far most reliable because it is unambiguous. Pruvost et al. 2011 tested predomestic wild horse remains from Europe of both the Holocene and Pleistocene for a number of colour alleles. It showed that the basic colours all were bay, black and leopard spotted (the latter is not relevant for the topic of this post, so let’s ignore it) . No dilution allele was detected within the wild horse populations, but the dun mutation was not yet resolved, therefore it was impossible to distinguish between dun and non-dun horses .
Historic references describe a variety of colours. They are less clear because there are numerous words to describe one and the same colour, translation errors are another factor. Eyewitness accounts often are ambiguous because the colour terms we use colloquially are not congruent with the terminology of coat colour genetics: mouse-coloured might refer to shades of bay, bay dun or black dun, but in coat colour genetics it is used only for black dun. That means we have to be cautious when interpreting historic accounts.
Another problem is that we don’t know for sure the status of the free-roaming horses described as “wild”. As I wrote in previous posts, there are good reasons to believe that many of the texts actually describe wild horses, but domestic introgression can never be ruled out. Therefore we should stick with what is likely to be a variation of those colour genetically proven to be predomestic, and consider other colours to be the result of intermixture.
So let’s have a look on what historic references actually say.
Herodot describes lightly coloured wild horses on the area of the Ukraine in the 5th century before Christ. “light” is quite vague, but it most likely means that the horses were dun-coloured. Albertus Magnus writes of mouse-coloured horses with a dark eel stripe in Germany of the 13th century. Balthazar Hacquet saw wild horses at the Zoo of Zamosc, Poland, in the 18th century and reported that they were of a blackish-brown colour. Kajetan Kozmian saw the wild horses of the same park and same century, and described them being uniformly of a dark mouse-colour. The reports of S. G. Gmelin and Peter Pallas refer to the wild horses of the Russian steppe (which also showed clear signs of intermixture), so they are of little value for the European forest horses .
The naturalist C. H. Smith described free-roaming “sturdy mountain-forest ponies” in his 19th century work The Natural history of horses, with memoir of Gesner (see here for his full description of western wild horses). These wild ponies were found in western Europe and according to him had “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” . Genuine wild horses extant in western Europe during the 19th century sound a bit far-fetched, but all the behavioural and phenotypic features Smith describes for these horses imply that they were wild or at least primitive feral ponies. The colour he describes certainly refers to bay dun (“yellowish brown and streak along the spine”) and black dun (“tendency to ashy and gray”), and also Herodot’s text supports that European wild horses were dun-coloured.
Dark brown might either refer to dark expressions of black dun or to the seal brown (bay) colour of ponies like the Exmoor. We probably will never know which colour that actually was, but the mealy mouth of bay horses could be a hint – this white area around the mouth is pretty apparent on an otherwise brownish coat, therefore it might have been mentioned if it was there. No historic reference mentions it, what might support the view that these horses had a brownish expression of black dun; but this is just a pure speculation of mine. The term “mouse coloured” also leaves big room for speculation. It is probably hard to figure out which colour they actually intended to describe with that term. European mice species display various shades of brown and gray, and no-one can know if the writer actually had any of these particular species in mind when speaking of mouse-coloured horses or just used it in the colloquial sense, and this colloquial sense might also be influenced by time and region. So we really have to guess.
All in all, it seems that historic references suggest European wild horses were dun-coloured, so both the colour of the Przewalski’s horse and brownish Koniks appeared. Intuitionally I consider it quite likely that there were non-dun wild horses in Europe as well because they are perfectly camouflaged in forested and bushy landscapes. But to be sure we have to wait until the mutations on the dun locus are identified so that dun and non-dun wild horses can be genetically distinguished. It would also be very interesting to know if the At allele was present in predomestic horse populations as well.
 Pruvost et al.: Genotypes of predomestic horses match phenotypes painted in paleolithic works of cave art. 2011
 Tadeusz Jezierski, Zbigniew Jaworski: Das Polnische Konik. 2008.
 Smith, Charles Hamilton: The Natural history of Horses, with Memoir of Gesner.(1814/1866)