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Source: Mesohippus, Encyclopedia Britannica

An overview of Parahippus, from the Florida Museum of Natural History.

A neat little article about Pliohippus. Most notably, it discusses the preorbital fossae I’ve seen mentioned in association with several fossil horses, dipping into hypotheses about their possible functions. Check the link if you’re interested in that sort of thing

Evolutionary family tree of the horse

Source

Pliohippus - 12 mya, North America

Fossil horses from the Ashfall Fossil Beds in Nebraska, a site whise richness of mammal fossils is attributed to a large Yellowstone eruption. Horse remains are plentiful, with five genera known to be present. They are depicted above in the following order: Pseudohipparion, Neohipparion, Cormohipparion, Protohippus, and Pliohippus. The first three genera are three-toed, while the last two bear single hooves.

Source (article)

“Pliohippus, extinct genus of horses that inhabited North America during the Pliocene Epoch (5.3–2.6 million years ago). Pliohippus, the earliest one-toed horse, evolved from Merychippus, a three-toed horse of the preceding Miocene Epoch (23–5.3 million years ago). The teeth of Pliohippus are taller and more complexly folded than those of earlier horses; these features indicate a greater dependence on grazing than browsing for food. Because of its diet and its specializations for running, it is likely that Pliohippus lived on open plains.”