Horse
The horse adapted to survive in areas of wide-open terrain with sparse vegetation, surviving in an ecosystem where other large grazing animals, especially ruminants, could not. Horses and other equids are odd-toed ungulates of the order Perissodactyla, a group of mammals that was dominant during the Tertiary period. In the past, this order contained 14 families, but only three—Equidae (the horse and related species), Tapiridae (the tapir), and Rhinocerotidae (the rhinoceroses)—have survived to the present day.
Onager
The genus "Equus", which includes all extant equines, is believed to have evolved from "Dinohippus" via the intermediate form "Plesippus". One of the oldest species is "Equus simplicidens", described as zebra-like with a donkey-shaped head. The oldest fossil to date is about 3.5 million years old from Idaho, USA. The genus appears to have spread quickly into the Old World, with the similarly aged "Equus livenzovensis" documented from western Europe and Russia.