The two major groups of living hoofed mammals are the Artiodactyla, or cloven-hooved mammals; and the Perissodactyla, or odd-toed mammals. The former is by far larger of the two groups, with over a hundred living species, including such familiar animals as sheep, goats, camels, pigs, cows, deer, and antelopes. The Perissodactyla contains only seventeen living species, classified into three living sub-groups: the horses, the rhinoceroses, and the tapirs. In the early Cenozoic, perissodactyls were much more diverse, including the great extinct brontotheres and Indricotherium, the largest land mammal of all time.
Many hoofed mammals live in grasslands and savannahs. Ungulates have evolved features that are adaptive for life on open grasslands, in particular long legs to increase running speed. To lengthen the legs, ungulates evolved digitigrade locomotion: that is, they walk on their toes. The hoof of a horse or cow is anatomically an enlarged toe. Artiodactyls such as deer, sheep, and goats walk on two toes; perissodactyls walk either on three toes (rhinos, tapirs, some extinct horses) or on one toe (living horses). The remaining toes not used for walking are either reduced, as in pigs and tapirs, or completely lost, as in rhinos and most ruminants. Many ungulates have also evolved large, complexly grooved molar teeth to grind their food of grasses and other plants.
Visit DeerNet for information on conservation of wild ungulates and their kin. This site includes a list of all species and a number of photos. Or check out this list of fascinating facts about the well-known hoofed mammal Alces alces.