A grindylow (or sometimes grundylow) is a water spirit out of English folklore. They are thought to have first appeared in Yorkshire and Lancashire, where they were known to grab children who went too near the water. Although it is likely that their name comes to us as a form of Grendel, academics have classified them as a type of nixie, a water nymph.
They are reported as being uniformly hostile and of indeterminate size, often appearing about the size of a small child. They cannot leave the water, but have very long arms and elongated fingers, allowing them to grab prey on the banks of streams and marshes. They prefer to kill by drowning, although they are said to have quite sharp teeth and claws. While they are generally considered evil, there is no evidence that they kill for any reason other than to procure food.
The English countryside had a number of other hostile water spirits, including Peg Powler, Jenny Greenteeth, the selkie, and further north, the kelpie and puka. These are all currently believed to be extinct, presumably because they were unable to adapt to the development and pollution accompanying the industrial revolution.
Nowadays grindylows are best known for their appearance in the Harry Potter books and movies, particularly in Harry Potter and the Goblet of Fire, where they appear as small, horned creatures that swim quickly and latch onto unwary swimmers. These grindylows seem to prefer the deep waters rather than lurking near banks. They appear to travel in packs, and do not seem to feed primarily, or even often, on humans. They are, however, aggressive towards both wizards and muggles.