The grindylow, known best from the Harry Potter stories, both from Remus Lupin’s Defense against the Dark Arts classroom and from Fleur Delacour’s mishap in the lake during the second challenge of the Triwizard Tournament, is actually a water demon from folklore.
According to Harry, the grindylow is “A sickly green creature with sharp little horns had its face pressed against the glass, pulling faces and flexing its long, spindly fingers.” (Harry Potter and the Prisoner of Azkaban). Their fingers are particularly dangerous, apparently. In the Harry Potter universe, these little demons hate people of all sorts, both muggle and wizard alike. However, strangely enough, merpeople keep them as pets. They eat mostly fish and things, but do eat people if they can get their dangerous fingers around them. That’s why they’re classified as dark creatures and Lupin includes them in his class.
But where did Ms. Rowling get this idea for the nasty little critter? They’re a pale green water demon of English decent. Some even link them to Grendel from Beowulf. Like the Harry Potter version, legendary grindylows snatch people, especially children, from the edges of lakes and pull them into the water to eat them. In legends, they’re often associated with critters like hags and used in the same manner to frighten children away from lakes and other dangerous places.
Since then, they’ve become fairly popular in mythical-based video games and such. Ms. Rowling launched them to demon-famedom, but what’s next for these little dudes?