Equus is a genus of mammals in the family Equidae, which includes horses, asses, 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.