Muggles' Guide to Harry Potter/Magic/Red Cap

Overview
Red Caps are dwarf-like creatures that are found wherever blood has been spilled.

Extended Description
In the Muggle world, we find the Red Cap in Scottish folklore. The Red Cap is a thoroughly evil creature. He is a short, stocky old man with long gray hair and claws instead of hands. He lives on the Scottish Border in ancient ruins of castles, especially in those with a bloody history of war and murder. He owes his name to the fact that he wears a red hat, which is colored by the blood of his victims. Red Cap moves with remarkable speed, despite the fact that he wears iron boots. He can overcome even the strongest man, unless the intended victim remembers to quote a few words from the bible. (Encyclopedia Mythica; thanks to Elizabeth Quest for the tip)

The Red Cap is introduced to Harry by Professor Lupin, then teaching Defence Against the Dark Arts, but while we hear the name in passing, we do not hear any description of it. We also hear it mentioned later when Hermione is telling Professor Snape, then filling in for an ill Lupin, what topics they have covered. Finally, a Red Cap is featured as part of the final Defence Against the Dark Arts exam, again without significant description.