The evolution of the horse did not occur in a straight line. The modern Equus is only a small part of the once-presentequine species. Also, the process was not smooth and gradual. Different traits evolved at different rates, and they didn't always develop together. Some species of equines branched off from others, and the two species coexisted. Most horse species arose in North America.

This page will discuss a few of the various species of horses that arose over time.

Edwards, Elwyn Hartley.  The Ultimate Horse Book.  London:
     Dorling Kindersley Limited, 1991.

Ensminger, M. E.  Horses and Horsemanship.  Danville, Illinois: The 
     Interstate Printers & Publishers, Inc., 1977.

Henry, Marguerite.  All About Horses.  New York: Random House, 1962.