A pachyderm is a really big animal with really thick skin, like an elephant or a hippo.
If you break this word down to its parts, you see pachy which means thick and derm which means skin. It comes from the Greek, but was first used to describe a class of animals by a French naturalist in 1797. He believed that thick skinned animals all belonged to the same family and categorized them together. We now know to classify them according to different features, but the word remains both in use and descriptively useful.