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.
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.