Re: Is a whale considered a pachyderm?

No – your son is right that "pachy-derm" translates as "thick-skinned", but the word, pachyderm, is actually a back- formation from the term, Pachydermata. The Pachydermata were an order of mammals from the classification system of Georges Cuvier, which he described in his Animal Kingdom: "They [sc. hoofed animals] can hardly be divided into more than two orders, those which ruminate, and those which do not; but these latter, which we designate collectively by the term Pachydermata, admit of subdivision into families. The first is that of the Pachydermata which have a proboscis and tusks." Conventional usage of pachyderm has since shifted to exclude all but elephants, rhinoceroses, and hippopotamuses; however, it still describes a paraphyletic (i.e., containing members from unrelated groups, while excluding members of related groups) group of mammals, and so is no longer an accepted zoological classification. Since it historically did not include whales, and since it is already to diverse to be valid, adding whales to the pachyderms seems unjustified.