"The Quintapeds cannot talk and have strenuously resisted every attempt by the Department for the Regulation and Control of Magical Creatures to capture a specimen and try to untransfigure it, so we must assume that if they are indeed, as their nickname suggests, Hairy MacBoons, they are quite happy to live out their days as beasts."
-- Newt Scamander (FB)
Quintapeds are highly dangerous carnivores covered in thick reddish-brown hair, having five clubfooted legs and a low-slung body. They have a particular taste for humans. Quintapeds are found only on the Isle of Drear off the north tip of Scotland.
There is a fascinating legend about the origin of Quintapeds, which involves a feud between two wizarding clans, the MacBoons and McCliverts, and a poorly thought-out magical attack. The McCliverts tried to be clever and turned the entire clan of MacBoons into Quintapeds, then realized they were so dangerous they could not be killed. The MacBoons killed every McClivert, leaving no one to transfigure the MacBoons back to human form, and thus they remain to this day. The Isle of Drear has been made Unplottable to protect the unwary since the Ministry has not be able to round up the Quintapeds (FB).
In the Room of Requirement, Harry discovered a cage which had the skeleton of a five-legged creature in it. Harry didn't recognize it, but this was a Quintaped (HBP24, JKR:Tw).
"quintaped" translates from Latin as "five-footed"
The quintaped has a fascinating backstory, the full of which can be found in Fantastic Beasts and Where to Find Them, and can be considered a warning against duels, drunkness, enimity and family rivalry, revenge, and not thinking things through.
J.K. Rowling confirmed the long held fan-theory that the skeleton of the five-legged creature that Harry discovered in the Room of Requirement in HBP24 belonged to a Quintaped on twitter.
From the Web
"Quintaped" from Harry Potter Wiki