They have a herbivorous diet, small brains 400–600 g (14–21 oz) for mammals of their size, one or two horns, and a thick 1.5–5 cm (0.59–1.97 in), protective skin formed from layers of collagen positioned in a lattice structure.

Unlike other perissodactyls, the two African species of rhinoceros lack teeth at the front of their mouths; they rely instead on their lips to pluck food.

[2] Rhinoceroses are killed by poachers for their horns, which are bought and sold on the black market for high prices, leading to most living rhinoceros species being considered endangered.

The contemporary market for rhino horn is overwhelmingly driven by China and Vietnam, where it is bought by wealthy consumers to use in traditional Chinese medicine, among other uses.

[9] A subspecific hybrid white rhino (Ceratotherium s. simum × C. s. cottoni) was bred at the Dvůr Králové Zoo (Zoological Garden Dvur Kralove nad Labem) in the Czech Republic in 1977.

[12] Some extinct rhinocerotids were considerably smaller and larger than living rhinoceroses, with the genus Menoceras from the Early Miocene of North America having an estimated body mass of 313 kilograms (690 lb),[12] comparable to sheep,[13] or a pig,[14] while Elasmotherium sibiricum from the Pleistocene of Eurasia has an estimated body mass of approximately 4,500 kilograms (9,900 lb).

[14] The brains of rhinoceroses are relatively small compared to body size,[18][19] around 531 grams (18.7 oz) in an adult black rhinoceros.

[14] The body is covered in an armour of thick skin made of a dense crosslinked network of collagen fibres that is stronger and stiffer than those of other mammals.

[21] The skin in living species is grey to brown in colour, and typically sparsely covered in hair or hairless as adults, with the exception of the eyelashes, ears, and the tail-brush.

Two-thirds of the world's Indian rhinoceroses are now confined to the Kaziranga National Park situated in the Golaghat district of Assam, India.

[44] These animals prefer dense lowland rain forest, tall grass and reed beds that are plentiful with large floodplains and mud wallows.

Though once widespread throughout Asia, by the 1930s, they were nearly hunted to extinction in Nepal, India, Burma, Peninsular Malaysia, and Sumatra for the supposed medical powers of their horns and blood.

Sumatran rhinoceros once were spread across South-east Asia, but now are on the verge of extinction, confined to several parts of Indonesia and Malaysia by reproductive isolation.

In 1994, Alan Rabinowitz publicly denounced governments, non-governmental organizations, and other institutions for lacking in their attempts to conserve the Sumatran rhinoceros.

To boost reproduction, the Malaysian and Indonesian governments could also agree to exchange the gametes of the Sumatran and (smaller) Bornean subspecies.

[54] Rhinocerotids represented the only living family of rhinocerotoids following the end of the Oligocene epoch around 23 million and the extinction of other rhinocertoid groups such as the giant paraceratheres.

South African officials have called for urgent action against poaching after poachers killed the last female rhino in the Krugersdorp Game Reserve near Johannesburg.

Hunting licenses for five Namibian black rhinos are auctioned annually, with the money going to the government's Game Products Trust Fund.

Esmond Bradley Martin has reported on the trade for dagger handles in Yemen,[97] which was historically a major source for the demand for rhino horn in the late 20th century.

[106] In 1993, China signed the CITES treaty and removed rhinoceros horn from the Chinese medicine pharmacopeia, administered by the Ministry of Health.

[citation needed] Vietnam reportedly has the biggest number of rhino horn consumers, with their demand driving most of the poaching, which has risen to record levels.

The ban was overturned in a court case in 2017, and South Africa plans to draft regulations for the sale of rhino horn, possibly including export for "non-commercial purposes".

[119] The South African government has proposed that a legal trade of rhino horn be established, arguing that this could reduce poaching and prevent the extinction of this species.

[120] In March 2013, some researchers suggested that the only way to reduce poaching would be to establish a regulated trade based on humane and renewable harvesting from live rhinos.

[121] The World Wildlife Fund opposes legalization of the horn trade, as it may increase demand,[122] while IFAW released a report by EcoLarge, suggesting that more thorough knowledge of economic factors is required to justify the pro-trade option.

Armed park rangers, particularly in South Africa, are also working on the front lines to combat poaching, sometimes killing poachers who are caught in the act.

Depending on the quantity of horn a person consumes, experts believe the acaricide would cause nausea, stomach-ache, and diarrhea, and possibly convulsions.

The original idea grew out of research into the horn as a reservoir for one-time tick treatments, and experts selected an acaricide they think is safe for the rhino, oxpeckers, vultures, and other animals in the preserve's ecosystem.

[137][138] Similarly in medieval era Thai literature, Agni also called Phra Phloeng is sometimes described as riding a rhinoceros.

[139][140] Albrecht Dürer created a famous woodcut of a rhinoceros in 1515, based on a written description and brief sketch by Valentim Fernandes, a German printer resident in Lisbon.