A kind of fish, shaped like a ball with two long, rubbery legs and webbed feet. The freshwater plimpy is a species of the plimpy, Xenophilius Lovegood uses it for soup.
If you happen to spot a plimpy with its legs tied in a knot, you will know that merpeople are around (FB). The stream near Bottom Bridge, behind the Lovegoods’ house, contains Freshwater Plimpies that Xenophilius uses to make soup (DH20).
"plim" Eng. a dialect word for becoming plump [NSOED]