Pliohippus is an extinct

Extinction

In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...

 genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of Equidae

Equidae

Equidae 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...

, the "horse family". Pliohippus arose in the middle Miocene

Miocene

The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

, around 12 million years ago, probably from Calippus

Calippus

Calippus may refer to:* Calippus of Syracuse, a student of Plato and Syracusean tyrant.* Callippus, a Greek astronomer and mathematician.* Calippus , a small lunar crater.* Calippus, an extinct relation of the modern horse....

. It was similar in appearance to Equus

Equus (genus)

Equus is a genus of animals in the family Equidae that includes horses, donkeys, and zebras. Within Equidae, Equus is the only extant genus. Like Equidae more broadly, Equus has numerous extinct species known only from fossils. This article deals primarily with the extant species.The term equine...

, 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

Steppe

In physical geography, steppe is an ecoregion, in the montane grasslands and shrublands and temperate grasslands, savannas, and shrublands biomes, characterized by grassland plains without trees apart from those near rivers and lakes...

 animal.
Until recently, because of its many anatomical similarities, Pliohippus was believed to be the ancestor of the present-day horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

 and its relatives in Equus. Although Pliohippus clearly is an equid and thus related to Equus, its skull had deep facial fossae, a feature not found in any member of Equus. Additionally, its teeth were strongly curved, unlike the very straight teeth of modern horses. Consequently, it is unlikely to be the ancestor of the modern horse; instead, it is likely to be the ancestor of Astrohippus

Astrohippus

Astrohippus is an extinct member of the Equidae tribe Equini, the same tribe that contains the only living equid genus, Equus. Fossil remains have been found in the central United States, Florida, and the Mexican states of Chihuahua, Jalisco, and Guanajuato...

. The Pliohippus was approximately six feet high, and weighed one thousand pounds. Its diet was plants. Its habitat was the plains of North America.

Fossils of the Pliphippus have been found at many late Miocene localities in Coarado, the Great Plains of the USA (Nebraska and the Dakotas) and also Canada. The Pliohippus has been found beside the Neohipparion.