"You are easily satisfied. Lupin is hardly over-taxing you - I would expect first-years to be able to deal with Red Caps and Grindylows."
-- Severus Snape PA9
Grindylows are found throughout Britain and Ireland. They can be found in the lake on the grounds of Hogwarts. Pale green in colour, these water demons have sharp horns, pointed fangs and brittle spindly fingers. Grindylows show aggression towards humans, wizards or Muggles, but merpeople have been known to domesticate them.
Grindylows first appear in the books in Prisoner of Azkaban when Lupin invites Harry into his office to have tea and to show Harry Grindylows, the creatures which will be the subject of their next Defense Against the Dark Arts lesson (PA8). Hermione tells Professor Snape that their class has been learning about Grindylows recently and he retorts that first years should be able to deal with them (PA9). Harry and the rest of his Defense Against the Dark Arts class have to wade across a pond containing a Grindylow as part of their exam (PA16). Harry and his fellow Triwizard champions have to get past Grindylows in the lake to retrieve what was taken from them during the Second Task of the Triwizard Tournament (GF26). Fleur Delacour was attacked by them and was unable to complete the second task (GF26).
The Grindylow is a mythical creature from taken from Yorkshire legends. The name is believed to have been derived from the words "grindel" and "low" (Colbert 6). The word "grindel", or "gryndel" in Middle English, is compared to the Old Norse word "grimd" which means fierceness (OED Online). "Low" relates to a "distance above or below a base level", but it also can mean "Of a river, the sea, etc.: below the usual water level; particularly shallow" (OED Online).
Colbert, David. "The Hidden Myths in Harry Potter." New York: St. Martin's Griffin, 2005. Print.
"grindel, adj." OED Online. Oxford University Press, June 2015. Web. 15 August 2015.
"low, adj. and n.2." OED Online. Oxford University Press, June 2015. Web. 15 August 2015.