Horse
Besides the horse, there are six other species of genus "Equus" in the Equidae family. These are the ass or donkey, "Equus asinus"; the mountain zebra, "Equus zebra"; plains zebra, "Equus quagga"; Grévy's zebra, "Equus grevyi"; the kiang, "Equus kiang"; and the onager, "Equus hemionus".
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.