I think it's a grindylow, too. Here's the description of one from the lexicon:

A grindylow is a pale green creature that lives in the weed beds on the bottom of lakes in Britain. It is also known as a water demon. Grindylows have long, brittle fingers which they use to grip their prey, sharp little horns, and green teeth. Lupin taught his third year students about them (PA8)

Grindylows in the lake near Hogwarts attacked the Triwizard champions during the second task. However, some grindylows appear to have been domesticated by merpeople.