Ghouls are slimy, buck-toothed, ugly creatures which live in the attics or barns of wizards. They are dim-witted and are content to throw things around now and then. Ghouls are relatively harmless creatures who live on spiders and moths (FB).
A ghoul lives in the attic of the Burrow, just above Ron's room. The ghoul makes noises a lot (CS4, GF10) . When Ron was about to leave home to look for Horcruxes, Fred, George and Arthur Weasley transformed the ghoul to look like Ron with Spattergroit (DH6).
There was a ghoul in the upstairs toilet at 12 Grimmauld Place (OP6).
The Ghoul Task Force in the Department for the Regulation and Control of Magical Creatures removes ghouls from Muggle homes (FB).
From Arabic "ġūl", a demon said to rob graves and devour corpses.