Originally, in Cuvier's system of classification: an order of mammals comprising elephants and related animals, and hoofed quadrupeds that do not chew the cud such as rhinoceroses, hippopotamuses, pigs, horses, etc. (now historical). In later use (treated as plural, in form pachydermata): animals belonging to this order; pachyderms.
Early 19th century; earliest use found in William Buckland (1784–1856), geologist and dean of Westminster. From scientific Latin Pachydermata, order name from ancient Greek παχύδερμος thick-skinned (Aristotle), after French pachyderme.