Equidae
Equidae (sometimes known as the horse family) is the taxonomic family of horses and related animals, including the extant horses, donkeys, and zebras, and many other species known only from fossils. All extant species are in the genus "Equus". Equidae belongs to the order Perissodactyla, which includes the extant tapirs and rhinoceros, and several extinct families.
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.