Pachyderm (meaning 'thick skin', from the Greek , and ) is a word used to group together elephants, rhinoceroses, tapirs, and hippopotamuses, four similar families of mammals. In the 1800s, many scientists believed that these four families of animals were closely related to each other, and that they were the only living descendants of some common ancestor species, but that theory was later proven wrong. Because of that, scientists who study the the relationships between animal species have mostly stopped using the word "pachyderm", but other people sometimes still use it as an informal category.