Equus (genus)
Equus is a genus of mammals in the family Equidae, which includes horses, donkeys, and zebras. Within Equidae, "Equus" is the only recognized extant genus, comprising seven living species. The term equine refers to any member of this genus, including horses. Like Equidae more broadly, "Equus" has numerous extinct species known only from fossils. The genus most likely originated in North America and spread quickly to the Old World. Equines are odd-toed ungulates with slender legs, long heads, relatively long necks, manes (erect in most subspecies), and long tails. All species are herbivorous, and mostly grazers, with simpler digestive systems than ruminants, but able to subsist on lower-quality vegetation.
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.