Wikipedia
Grindylow
A grindylow or grundylow is a folkloric creature that originated from folktales in the English counties of Yorkshire and Lancashire. The name is thought to be connected to Grendel, a name or term used in Beowulf and in many Old English charters where it is seen in connection with meres, bogs and lakes.
Grindylows are said to grab little children with their long sinewy arms and drown them if they come too close to the water's edge. Grindylows have been seen as a bogeyman used as a ploy to frighten children away from pools, marshes or ponds where they could drown.
Peg Powler and Jenny Greenteeth are similar water spirits.