Living perissodactyls are remnants of a large and varied group that flourished in the Tertiary Period (66.4 to 1.6 million years ago). Eight entire families known from the fossil record are extinct, in addition to numerous species from the three families still extant. The fossil history of the order is well-known, especially in the case of Equidae, making their development of great scientific interest and a significant force shaping evolutionary thought. The perissodactyls are large, ranging from the 600-pound (270-kilogram) tapir to the 4,500-pound (2,040-kilogram) rhinoceros. All perissodactyls feed by grazing or browsing, and their teeth have evolved specialized forms adapted to grinding. Various living species have seen their numbers reduced by hunting and human expansion; many are presently protected in game reserves and national parks.

Horses, asses, and zebras (see photograph ) adapted to a variety of environments, although not forests or other areas of dense vegetation. They are a highly specialized running form, subsisting almost entirely on grasses. Two races of wild horse (Equus caballus), one gray (the tarpan) and one reddish brown (e.c. Przewalskii), were once widely distributed throughout northern Eurasia. The tarpan became extinct in Ukraine in the mid-19th century, although tarpanlike horses exist today as a result of selective breeding of domestic horses known to have tarpan ancestry. In addition, Przewalski's horse, thought to have disappeared as a truly wild animal before 1950, may still exist in the remote semidesert lands near the Mongolian-Chinese border. The domestic horse, a descendant of the wild horse, is found throughout the world.

Three species of zebra are found on the African continent, ranging from mountainous terrain to sparsely wooded savanna. Originally very numerous, their numbers have also been severely reduced (one variety now numbers perhaps 100 animals) primarily by competition from domestic livestock for meagre pastureland. A large proportion of zebras are to be found in reserves or enjoying legal protection.

The true ass (Equus asinus), found in arid North Africa, is the ancestor of the modern donkey. It is probable that truly wild asses no longer exist, the strain having been diluted by interbreeding with escaped or released domestic donkeys. All races of the half-asses (E. hemionus) that are not extinct are rare. The species, which includes the onager, one of which stands only 1 m (3 feet) tall at the shoulder, ranges from Syria through Central Asia to Mongolia.

There are two living species of rhinoceros found in Africa and three in Asia. As with the Equus family, all species have been severely reduced in number, some presently being close to extinction, and many are protected in reserves and parks. All are characterized by very large size, extremely thick and mainly hairless hides, and one or, in the case of the Indian rhinoceros, two horns. The African species are the black, or prehensile-lipped, rhinoceros and the white, or square-lipped, rhinoceros. The black rhinoceros, ranging from southern Africa to as far north as the Côte d'Ivoire, is a selective browser and can apparently survive without free water by devouring succulent plants. Its numbers have been reduced mainly by poachers seeking the horns. The much larger white rhinoceros is mainly a grazing animal whose numbers have been reduced to the approximately 1,700 confined to the game reserves of Zululand and an additional population in Congo (Kinshasa).

Less than 1,000 members of the three Asian rhinoceroses survive today in widely separated pockets. The Sumatran rhinoceros, the smallest of the family, standing 1 to 1.5 m (3 to 5 feet) at the shoulder, is thought to number between 100 and 170. Only 25 to 60 Javan rhinoceroses survive, confined to a single reserve in western Java. The most numerous species, the Indian, or one-horned, rhinoceros, is threatened by hunting and by expanding human populations.

The tapirs are rounded, semiamphibious, piglike creatures that inhabit forests and woodlands, subsist by browsing, and are characterized by a trunklike snout and a coat of short, bristly hair. One Asian species (Tapirus indicus), the largest tapir, is found in Sumatra and the Malay peninsula, while three species, the mountain (T. pinchaque), Baird's (T. bairdii), and the Brazilian (T. terrestris), range throughout Middle and South America. The mountain tapir is the smallest and most primitive species and is found in Colombia and Ecuador at altitudes up to 15,000 feet. Baird's tapir, the largest, extends from Mexico to coastal Ecuador. Its shyness and difficulty in adjusting to human settlement have greatly reduced its range and numbers.

All ungulates, including the perissodactyls, were probably derived from the order Condylarthra, unspecialized tapir-sized mammals, and emerged 55,000,000 to 40,000,000 years ago, during the Eocene. The earliest horses (formerly Eohippus, now known as Hyracotherium) were browsing, forest-dwelling creatures, sometimes no larger than a fox terrier, that appeared in Europe and North America. Pliohippus, the line from which modern horses are descended, developed in North America during the Pleistocene and then spread throughout most of the world. One group of animals (e.g., Homogalax) was the common ancestor of both the tapirs and rhinoceroses. After the separation of the families, some rhinoceros ancestors developed massive horns, and one, Baluchitherium, grew to stand 5.5 m (18 feet) at the shoulder.

The perissodactyls as a rule avoid human contact and play little role in the affairs of man. The exceptions, of course, are the equines, particularly the horse and ass, whose importance in the history of mankind as pack, draft, and riding animals, as well as occasionally as food, is very great indeed.