Horse

Horse, hoofed, herbivorous mammal (genus Equus). This is the only living genus of the family Equidae; the donkey and the zebra are different species of the same genus. Wild horses occurred in prehistoric times over most of Eurasia. Today, the only surviving true wild horse is the Przewalski horse of Siberia, Mongolia, and western China. Domestic horses (E. caballus) can be grouped as ponies, heavy draft horses, lightweight draft, and riding horses; they originated from North African stock. Thoroughbreds are descended from Arabian stock. The fossil record of the horse family provides a classic example of evolution in action. The earliest animal that can be placed in the family was Eohippus, which was the size of a fox terrier and had 3 toes of equal size on each hindfoot and 4 toes on each forefoot. The development of the single-toed foot of modern horses was an adaptation to running on hard dry grassland. The changes in tooth pattern allowed the animal to eat abrasive grasses. The increase in size may be followed through a continuous series of intermediate stages to the present day. The various species of the genus Equus can often interbreed (e.g., horses and donkeys), but the offspring (e.g., mules) are generally sterile.