Until recently, because of its many anatomical similarities, Pliohippus was believed to be the ancestor of the present-day horse 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. Pliohippus stood approximately 1.25 metres, similar to the modern horse. Also like the modern horse, Pliohippus was a grazer that fed of steppe grasses of the North American plains it inhabited.
Fossils of the Pliohippus have been found at many late Miocene localities in Colorado, the Great Plains of the USA (Nebraska and the Dakotas) and also Canada. The Pliohippus has been found beside the Neohipparion.