Equines originated between 60 and 50 million years ago with the cocker spaniel-sized Eohippus, or Dawn Horse. This had four padded toes on the front legs and three on the back, which allowed easy movement over wet ground. Eohippus browsed on the leaves of low-growing shrubs and lived in the semi-tropical forests of the U.S. Midwest.
Eohippus evolved to be taller and heavier, with teeth that allowed it to eat a wider variety of plants. The front feet were reduced to three toes.
About 26 million years ago, the horse moved out of the forests and swamps and onto the plains. Its neck and head became longer, the incisors moved forward in the skull and the form and position of the eyes altered to allow the horse to view the horizon while grazing. Its legs became longer, giving it speed to escape from predators and it stood on a single toe.
Equus caballus, which evolved almost two million years ago, was the first true horse. It moved across the Bering Strait from America to Asia. Primitive man followed horse herds back across the Bering Strait into America, some staying to become the first Americans. When the glaciers retreated ten thousand years ago, the land bridges between what is now Alaska and Asia disappeared. Soon after that the horse became extinct in North America. They were later re-introduced to the continent by Spanish explorers, giving rise to the feral Mustang.
The indigenous peoples of the Americas did not have a specific word for horses, and referred to them as a type of dog or deer (in one case, "elk-dog").