Muggles' Guide to Harry Potter/Magic/Grindylow

Overview
The Grindylow is a pale green water demon. It is found throughout lakes in Britain and Ireland, including the lake at Hogwarts. It acts with aggression towards muggles and wizards alike, but Merpeople have been known to domesticate it. Grindylows have long fingers which will allow them to get a good grip, but which are easy to snap.

Extended Description
The Grindylow appears first in Harry Potter and the Prisoner of Azkaban, where Professor Lupin brings it in for his Defence Against the Dark Arts classes. We see it only in passing there, being used only to illustrate Professor Lupin's willingness to communicate with the students in general, and Harry in particular. It reappears as part of the final exam for that course.

The Grindylow reappears in Harry Potter and the Goblet of Fire, as one of the obstacles that Harry must pass in order to complete the Second Challenge. It is in this section that we see that the Grindylow has been domesticated by merpeople; there are some tied up, like dogs, near some of the merpeople's houses.

The Grindylow is also mentioned in Harry Potter and the Deathly Hallows, in order to confirm that it is Harry he is talking to, Lupin asks Harry what was in Lupin's office the first time Harry was in there, and Harry replies, correctly, that it was a Grindylow in a tank.