Fossils of Eohippus, as the first horses have been called, showed the mammal to be an herbivore smaller than a dog. Eohippus lived primarily in North America but vanished from the continent entirely around 8,000 years ago for reasons that remain a mystery. Horses did not return to North America until the fifteenth century A.D.

Changing habitat from swamplands to dry savannahs caused the horse to evolve from a creature with multiple toes to one with a single toe, which later became a hoof and which is better adapted to roaming dry ground. We can thank the Pliocene epoch for Pliohippus, the first single-toed horse. Pliohippus serves as a prototype for our own Equus, the modern horse. According to The Kingdom of the Horse, by Elwyn Hartley Edwards, Pliohippus had a ligament-sprung hoof and longer legs with flexing ligaments, which gave way to a running action similar to that of the modern horse.

The evolution from Pliohippus to our modern Equus took another 5 million years or so to accomplish. Due to changes in climate and landmass during that time, the early horse found its way from North America to South America and spread across Asia, Europe, and Africa.