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.
Pliohippus
Pliohippus (Greek πλεῖον (pleion, "more") and ἵππος (ippos, "horse")) is an extinct genus of Equidae, the "horse family". "Pliohippus" arose in the middle Miocene, around 15 million years ago. It was similar in appearance to "Equus", but had two long extra toes on both sides of the hoof, externally barely visible as callused stubs. The long and slim limbs of "Pliohippus" reveal a quick-footed steppe animal.