The last real tarpan (from the photo) died in 1860, and they've tried to breed them back with mixed success. The heck horse is quite similar (and I think the best result), it's supposed to be mouse grey, with (partly) falling black manes, stocky, sturdy animal, 130cm high (its back). The przewalski is the eastern wild horse that just got saved in time. The latter has the upright manes and the white around the nose, the tarpan shouldn't have that (and I removed it from my textures in the meantime).
I was just thinking about painting my render (in oils) for an upcoming painting contest, TG is a quite handy tool actually to arrange stuff and manipulate light and textures for paintings. Here's another one I want to paint, which was quickly put together just now for testing its potential. A bird will sit on the front sticks.