Simply put, hoofed mammals have toes covered with a horny structure composed of keratin that helps them to run away from predators efficiently. Hooves, like the nails of primates, evolved from the keratinous claws of other mammals, such as cats and rodents. Like ballet dancers standing on point, these animals have their entire weight concentrated on their toes.
Hoofed mammals have traditionally been called ungulates (from the Latin ungula, “hoof”), a group that had a common origin sometime in the Late Cretaceous, 70–65 million years ago. Recent evidence that also places the nonhoofed Elephants, Hyraxes, and aardvarks in the Superorder Ungulata (as well as the aquatic Cetaceans and Sirenians derived from ungulates) makes it more convenient to group hoofed cryptids separately. Most are herbivorous.
The two major extant orders of hoofed mammals are:
(1) The Artiodactyla, the order of even-toed or cloven-hoofed animals that includes cattle, deer, antelopes, giraffes, pigs, hippos, and camels. First seen in the Early Eocene, 55 million years ago, they are characterized by their elongated third and fourth toes, which form the primary support for the limbs. The skulls of living artiodactyls are elaborately modified for defense, with canines, incisors, horns, and antlers.
(2) The Perissodactyla, the order of odd-toed animals, with the middle toe bearing the primary weight. These animals include horses, rhinos, tapirs, and the extinct chalicotheres and brontotheres. This group diversified in North America and Eurasia to become the most abundant herbivores between 55 and 25 million years ago.
Extinct orders of hoofed mammals are the embrithopods of Oligocene Africa, which included the rhinolike, twin-horned Arsinoitherium; the notoungulates, South American ungulates that included the horse- or rhinolike toxodonts and the smaller typotheres; the litopterns, also endemic to South America, which incorporated the long-necked, camel-like macraucheniids; the uintatheres of North America and Asia, among them the huge Uintatherium, which had three pairs of bony swellings on its skull and powerful canine teeth; the carnivorous mesonychids such as Andrewsarchus that may have been ancestral to cetaceans; the astrapotheres, South American animals that resembled tapirs or rhinos; and the pyrotheres and xenungulates, little-known South American ungulates.
Of the thirty-eight mystery animals in this list, twenty seem related to pigs, hippos, camels, deer, antelopes, giraffes, or oxen; eleven can apparently be grouped with horses, rhinos, and tapirs; three may be surviving notoungulates; one could be a survival into historical times of a litoptern; and three are too problematic to classify. Six are found in North America, six in South America, four in Europe, eight in Africa, twelve in Asia, and two in Australasia.