The Perissodactyla group is referred to as odd-toed ungulates, having four toes on each front foot and three toes on each hind foot
Today the mammalian order Perissodactyla, also often referred to as odd-toed ungulates, having four toes on each front foot and three toes on each hind foot, consist of 17 extant species that are derived from three separate families. These families are the Equidae (horse, zebras and asses), Rhinocerotidae (rhinoceros) and Tapiridae (tapirs) (Norman & Ashley, 2000).
As a group Perissodactyla arose during the late Palaeocene or early Eocene (Norman and Ashley, 2000); this group resulted from major adaptive shifts, specifically the ankle joint that limit side movement, and a change in dental formation, particularly their molar pattern along with the jaw muscles (Radinsky, 1969).
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Norman, J.E. & Ashley, M.V. (2000). Phylogenetics of Perissodactyla and Tests of the Molecular Clock. J. Mol. Evol. 50, 11–21.
Randinsky, L. (1969). The Early Evolution of the Perissodactyla. Evolution, 23(2), 308-328.