Western Washington has been invaded by a South American army ... of nutria (Myocastor coypus). Nutria are small (12- to 17-inch) semi-aquatic rodents which look like a beaver on steroids. Protectors of native species are organizing to repel the invasion. The nutrias' plan is to win by procreation (about sixty offspring per female per year).

The name of this wee beastie looks, for all the world, like a plural to me. The back formation of the singular would then be "nutrium" but that is unlikely. Where'd these critters get such a name?