Muggles' Guide to Harry Potter/Magic/Plimpy

Overview
Plimpies are apparently aquatic, but are never described in the series, though they are mentioned by both Luna Lovegood and her father Xenophilius.

Extended Description
In Harry Potter and the Half-Blood Prince, we learn from Luna Lovegood that Gulping Plimpies can be staved off by means of Gurdy-roots.

In Harry Potter and the Deathly Hallows, Xenophilius Lovegood explains Luna's absence by claiming that she is down on the far side of the bridge, catching Plimpies for their supper. He says that many people have asked for the recipe for their Plimpy soup.

In Magical Beasts And Where To Find Them, there is a description of Plimpies. Evidently these are approximately spherical fish with legs and feet. It is mentioned that Merpeople can get irritated by their habit of hovering around, and will tie their feet together. A Plimpy with its feet tied together will drift helplessly until it manages to get its feet untied, which can take hours.