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.
Mustang
The taxonomic horse family "Equidae" evolved in North America 55 million years ago. By the late Pleistocene era, there were two species of the family remaining there, the caballine (stout legged) and stilt-legged, which recent DNA studies have indicated represent different genera; ""Equus"" and ""Haringtonhippus,"" respectively. "Haringtonhippus" went extinct, and "Equus" was extirpated from the Americas at the end of the last ice age, possibly due to a changing climate or the impact of newly arrived human hunters. Thus at the beginning of the Columbian Exchange, there were no equids in the Americas.