Most of the evolutionary development of the horse (54 million years ago to about 10,000 years ago) actually took place in North America, where they developed the very successful strategy of grazing (eating grass) rather than browsing (eating softer succulent leaves). [...]
Horses (Equus)continued to evolve and develop for another six million years after Pliohippus and became very successful, spreading throughout North America. At some point some of them crossed into the Old World via the Arctic-Asia land bridge. Then, suddenly, no one is absolutely certain why, between 10,000 and 8,000 years ago, Equus disappeared from North and South America. Various theories have been advanced including destruction by drought, disease, or extinction as a result of hunting by growing human populations. At any rate, the horse was gone from the western hemisphere. The submergence of the Bering land bridge prevented any return migration from the Old World or Asia, and the horse was not seen again on its native continent until the Spanish explorers brought horses by ship in the sixteenth century.