The last remnant population of aurochs was found in a Polish game reserve in possession of the Polish crown from the 16th century until their extinction in 1627. From about 1600 onwards, the last aurochs were found exclusively in the forest of Jaktorow. It was the first extinction of an animal that was precisely documented. In 1564, the number of living aurochs was already as low as 38. In 1599, there were only 24 aurochs left. The 1602 mentions only four individuals: three bulls and one cow. In 1620 it was reported that the last aurochs bull had died and only one cow was left. In 1630, it was noted that this very last cow died three years ago. I illustrated her presumable lone death with a drawing and a little story here in 2015. The reasons why the last aurochs could not survive were poaching and hunting, cattle diseases, limited space and food as much as several severe winters the aurochs could not cope with in their strongly restricted habitat.
It is a myth that the last individual was hunted or poached, it is also a Wikipedia fabrication that her skull was preserved and sent to the Royal Armoury of Stockholm. Unfortunately no skeletal remains are known from these very recent aurochs. But horns and skins of aurochs from Jaktorow were indeed preserved and ornamented and sent to the nobility, and so was one of the horns of the last aurochs bull.
Some weeks ago I came across high-resulted images of this horn from several useful angles on Wikipedia provided with detailed information by the Royal Armoury of Stockholm.
What is very apparent at first is that the horns are comparably thin and slim, fitting a notion by Anton Schneeberger who visited the Jaktorow population in the 16th century. Aurochs horns from earlier centuries are noticeable thicker and also more curved (for example see here, a photo of medieval drink horns at the National Museum of Copenhagen that might be of aurochs, by Markus Bühler). The curvature seems indeed very meagre also compared to most bony core fossils (which, logically, were still less curved than their horn sheaths that covered them in life). See here, for example.
Even more surprising are the size data for the horn given in the description. Apparently it measures only 34,8 cm in length (!) and only 5,7 cm in diameter. I assume that the bull was fully grown when it died, and even if it was a subadult bull, the horn must have been still quite small proportionally for aurochs standards. It is possible that the overall size of the last aurochs was smaller after all, but Schneeberger still described them as way larger than domestic cattle.
The horns of the last aurochs bull might have resembled those of this Maronesa bull, though probably not oriented as lowly.
Why were the horns of the last aurochs so small and meagre compared to those of earlier centuries and millennia? I see two major factors. One would be trophy hunting. Aurochs horns were regarded as prodigious trophies, and hunting for aurochs with particularly large and impressive horns would be directive selective pressure against such horns, eventually resulting in the horns becoming smaller and less impressive. This effect of trophy hunting has also been observed in elks and African elephants, although I have no written source at hand at the moment. The other and probably more important factor would be the limited habitat and therefore limited food supply. This would have led to a stint in horn volume, especially if the mineral supply (calcium in particular), was low. Perhaps the small size of the horns was partially due to phenotypic plasticity and they would have grown larger horns if they had grown up on an area of sufficient food and mineral supply, but I consider it very likely that much of shrinkage in horn size also had a genetic component.
In any case, it of course provokes the question if the “breeding-back” standards should also include this meagre horn shape and size. I do not recommend it for two reasons. At first, the causes of the meagre horns of the last aurochs remnants were likely anthropogenic. Furthermore, most “breeding-back” results have horns smaller and less curved than the overwhelming maturity of all aurochs populations of all ages and regions, and permitting horns as small and weakly curved as that of the last aurochs bulls might make it even more difficult to move the average horn type found in “breeding-back” results towards what was typical of most aurochs.
Cis van Vuure: Retracing the aurochs: history, morphology and ecology of an extinct wild ox. 2005.