"ungainly quadruped having tough, thick skin and usually one or two horns on the snout," once widespread but now limited to Africa and South Asia, c. 1300, rinoceros, "a rhinoceros," also "a horned beast, sometimes regarded as a species of unicorn" [Middle English Compendium], from Latin rhinoceros, from Greek rhinokerōs, literally "nose-horned," from rhinos "nose" (a word of unknown origin) + keras (genitive keratos, kerōs) "horn of an animal" (from PIE root *ker- (1) "horn; head").
What is the plural of rhinoceros? ... Well, Liddell and Scott seem to authorize 'rhinocerotes,' which is pedantic, but 'rhinoceroses' is not euphonious. [Sir Charles Eliot, "The East Africa Protectorate," 1905]
Medieval Latin used rhinocerota. The adjective also is unsettled: Candidates include rhinocerotic, rhinocerical, rhinocerontine, rhinocerine. The rhinoceros beetle (by 1680s) is so called for the large, upcurved horn on the head of males.