Perissodactyla in the context of "Unguligrade"

Equidae (commonly known as the horse family) is the taxonomic family of horses and related animals, including asses, zebras, and many extinct species known only from fossils. The family evolved more than 50 million years ago, in the Eocene epoch, from a small, multi-toed ungulate into larger, single-toed animals. All extant species are in the genus Equus, which originated in North America. Equidae belongs to the order Perissodactyla, which includes the extant tapirs and rhinoceros, and several extinct families. It is more specifically grouped within the superfamily Equoidea, the only other family being the extinct Palaeotheriidae.

The term equid refers to any member of this family, including any equine.

Ungulates are members of the diverse clade Euungulata, which primarily consists of large mammals with hooves. Once part of the taxon "Ungulata" along with paenungulates and tubulidentates, as well as several extinct taxa, "Ungulata" has since been determined to be a polyphyletic grouping based on molecular data. As a result, true ungulates had since been reclassified to the newer clade Euungulata in 2001 within the clade Laurasiatheria, while Paenungulata and Tubulidentata had been reclassified to the distant clade Afrotheria. Alternatively, some authors use the name Ungulata to designate the same clade as Euungulata.

Living ungulates are divided into two orders: Perissodactyla including equines, rhinoceroses, and tapirs; and Artiodactyla including cattle, antelope, pigs, giraffes, camels, sheep, deer, and hippopotamuses, among others. Cetaceans such as whales, dolphins, and porpoises are also classified as artiodactyls, although they do not have hooves. Most terrestrial ungulates use the hoofed tips of their toes to support their body weight while standing or moving. Two other orders of ungulates, Notoungulata and Litopterna, both native to South America, became extinct at the end of the Pleistocene, around 12,000 years ago.

Equoidea is a superfamily of hippomorph perissodactyls containing the Equidae, Palaeotheriidae, and other basal equoids of unclear affinities, of which members of the genus Equus are the only extant species. The earliest fossil record of the Equoidea proves unclear, but they possibly could have originated during the late Paleocene in Europe or Asia. Definite fossil records of equoids are recorded by the earliest Eocene, in which the earliest equids in North America and basal equoids of unclear affinities in Europe both appeared. Palaeotheres are thought to have originated later in the early Eocene of Europe, although some researchers disagree on whether the subfamily Pachynolophinae belongs to the Palaeotheriidae.

Equoids may in part be defined by dental synapomorphies from its molars. Equoids were originally brachyodont (low-crowned) in dentition, but the morphologies of their molars had changed to adapt to different diets. Living equids are hypsodont, the result of evolutionary adaptations towards grazing diets.

Tapirs (/ˈteɪpər/ TAY-pər) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a pig, with a short, prehensile nose trunk (proboscis). Tapirs inhabit jungle and forest regions of South and Central America and Southeast Asia. They are one of three extant branches of Perissodactyla (odd-toed ungulates), alongside equines and rhinoceroses. Only a single genus, Tapirus, is currently extant. Tapirs migrated into South America during the Pleistocene epoch from North America after the formation of the Isthmus of Panama as part of the Great American Interchange. Tapirs were present across North America, but became extinct in the region at the end of the Late Pleistocene, around 12,000 years ago.

Laurasiatheria (/lɔːrˌeɪʒəˈθɪəriə, -θɛriə/; "Laurasian beasts") is a superorder of placental mammals that groups together true insectivores (eulipotyphlans), bats (chiropterans), carnivorans, pangolins (pholidotes), even-toed ungulates (artiodactyls), odd-toed ungulates (perissodactyls), and all their extinct relatives (pan-euungulates). From systematics and phylogenetic perspectives, it is subdivided into order Eulipotyphla and clade Scrotifera. It is a sister group to Euarchontoglires with which it forms the magnorder Boreoeutheria. Laurasiatheria was discovered on the basis of the similar gene sequences shared by the mammals belonging to it; no anatomical features have yet been found that unite the group, although a few have been suggested such as a small coracoid process, a simplified hindgut (reversed in artiodactyls), high intelligence, lack of grasping hands (though mimicry of grasping is observed in felines) and allantoic vessels that are large to moderate in size. The Laurasiatheria clade is based on DNA sequence analyses and retrotransposon presence/absence data. The superorder originated on the northern supercontinent of Laurasia, after it split from Gondwana when Pangaea broke up. Its last common ancestor is supposed to have lived between ca. 76 to 90 million years ago.

Brontotheriidae is a family of extinct mammals belonging to the order Perissodactyla, the order that includes horses, rhinoceroses, and tapirs. Superficially, they looked rather like rhinos with some developing bony nose horns, and were some of the earliest mammals to have evolved large body sizes of several tonnes. They lived around 56–34 million years ago, until the very close of the Eocene. Brontotheres had a Holarctic distribution, with the exception of Western Europe: they occupied North America, Asia, and Eastern Europe. They were the first fossilized mammals to be discovered west of the Mississippi, and were first discovered in South Dakota.