Muggles' Guide to Harry Potter/Magic/Erumpent

Overview
An Erumpent is an animal that grows at least one large, spiral horn.

Extended Description
While we never see an Erumpent, we do see its horn. The horn is extremely large, leading us to believe that the animal in question is also very large. The horn in question is in Xenophilius Lovegood's study, where he has purchased it believing it to be the horn of a Crumple Horned Snorkack.

Hermione recognizes it at once, and tells Xenophilius that it is extremely unstable; some time afterwards, a stray Stunner hits it, and it explodes, causing serious damage to Xenophilius' house.

The Erumpent is further described in Fantastic Beasts and Where to Find Them by "Newt Scamander" as a large grey beast, native to Africa, that can be mistaken at a distance for a rhinoceros. The horn is extremely sharp, able to penetrate metal, and contains an exploding fluid.