A kind of fish, shaped like a ball with two long, rubbery legs and webbed feet. The Gulping Plimpy is a subspecies of plimpy that is warded off by Gurdyroots.
If you happen to spot a plimpy with its legs tied in a knot, you will know that merpeople are around (FB). Luna carries a Gurdyroot to ward off Gulping Plimpies (HBP20).
"plim" Eng. a dialect word for becoming plump [NSOED]