The origin of equines can be traced to the Eocene period, between 60 and 50 million years ago. Eohippus, or Dawn Horse, was about the size of a Cocker Spaniel - 14 inches at the shoulder - and is thought to have weighed about twelve pounds. He had four toes on the front legs and three on the back, which were padded like those of a dog and allowed easy movement over wet ground. These toes and pads are now the ergots and splint bones found on the legs of the modern horse. Eohippus was a browsing animal that lived on soft leaves growing on low shrubs. He was well equipped to survive in what were then the semi-tropical forests of the U.S. Midwest.

By the Oligocene period, about 38 million years ago, Eohippus had evolved into Mesohippus and Miohippus and had achieved the size of a German Shepherd. Both these evolutions were taller and heavier, with teeth that allowed them to eat a wider variety of plants. They were still browsers living in forests and swamps. Their front feet were reduced to three toes, still padded, but the middle toe carried most of the weight.

The watershed in the development of the horse occurred in the Miocene period, about 26 million years ago, when he moved out of the forests and swamps and onto the plains. As he adapted to changing conditions, his 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. His legs became longer, giving him speed to escape from predators. These horses, Parahippus and Merychippus, stood firmly on a single toe with semi-functional side toes, and were about 10.5 hands (42") high.

The first truly single-hoofed horse was Pliohippus, which evolved about seven million years ago in the Pliocene period. The side toes became the splint bones found in modern horses. This small, lightly built horse was the prototype for the Equus caballus, the first true horse, which evolved during the Pleistocene period, almost two million years ago. Equus had a rigid spine, with short, powerful and well-muscled bones in the upper limbs and long, slender unmuscled lower limbs. He was well equipped for life on the open plain and had a well developed defense system. The foot pad of earlier evolutions became the frog of modern horses.

Equus spread across the Bering Strait from America to Asia. Primitive man, starting to evolve in Asia, followed horse herds back across the Bering Strait into America, some staying to become the first Americans. When the glaciers retreated about 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. No one knows why. They were later re-introduced to the continent by Spanish explorers, and became the progenitors of the Mustang.

information from EquiWorld