The earliest known direct ancestor of Equus, the eohippus [Gr.,=dawn horse], 10 to 20 in. (25–50 cm) tall, lived approximately 50 million years ago in both the Old and New Worlds. Equus originally evolved in North America by the late Pliocene epoch, about three million years ago, spreading to all continents except Australia. Horses disappeared from the Americas for unknown reasons about 10,000 years ago, to be reintroduced by Europeans, c.A.D. 1500.

Many species of Equus arose in the Old World. Horses were probably first domesticated by central Asian nomads around 3500 B.C. Horses were recorded in Mesopotamia and China (c.2000 B.C.), Greece (c.1700 B.C.), Egypt (c.1600 B.C.), and India (c.1500 B.C.). Horses were domesticated in W Europe no later than 1000 B.C. It is not known whether these early domesticated horses developed from a single wild race or from many local races.

Largely superseding the slower, less manageable ass, which had been domesticated much earlier, the horse's first known use was for drawing Mesopotamian war chariots. It was long reserved primarily for warfare and for transportation for the rich and well-born, while cheaper animals (e.g., oxen, mules, and donkeys) were used for lowlier work. Horses figured importantly in war and conquest in Europe, central Asia, and the Middle East for over 3,000 years. Early warriors rode bareback or with saddle cloths. The saddle and the stirrup were probably developed in China in the early Christian era, spread by Asian horsemen (such as the Huns), and adopted by Arabs and Europeans in the early Middle Ages. Arab cavalry conquered the Middle East and N Africa in the 7th cent. A.D. In the same period, armored knights were riding to battles in Europe. With highly developed cavalry tactics, the Mongols extended their 13th cent. empire from China to E Europe.

The Spanish conquistadors brought horses to the New World, where Native Americans soon acquired them from ranches and missions. The Plains Indians of North America quickly developed a horse culture that led to their ascendancy in numbers and power. Horses were used for hunting buffalo and other game, for warfare, and for pulling loads on a travois. Escaped Indian horses were ancestral to the mustang, the so-called wild horse of the W United States.

The two major groups of modern horses—the light, swift southern breeds, called light horses, and the heavy, powerful northern breeds, called draft horses—are believed to have arisen independently. The small breeds called ponies may derive from a southern, light horse or from a wild race.

Horses are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Perissodactyla, family Equidae.