"Ah, but the Third Hallow is a true Cloak of Invisibility, Miss Granger! I mean to say, it is not a traveling cloak imbued with a Disillusionment Charm, or carrying a Bedazzling Hex, or else woven from Demiguise hair, which will hide one initially but fade with the years until it turns opaque. We are talking about a cloak that really and truly renders the wearer completely invisible, and endures eternally, giving constant and impenetrable concealment, no matter what spells are cast at it. How many cloaks have you ever seen like that, Miss Granger?"
The Demiguise is a peaceful, herbivorous creature that can make itself invisible. It resembles an ape with large, black eyes and long, silky hair. This hair can be woven into Invisibility Cloaks. The Demiguise is native to the Far East (FB).
When describing the power of the second Deathly Hallow, a true Cloak of Invisibility, to Harry, Ron and Hermione Xenophilius Lovegood mentions that a travelling cloak woven with Demiguise hair will hide the wearer initially but fade with the years until it turns opaque. Such a cloak would make the wearer invisible. (FB)
Ug the Unreliable, a goblin shyster, organized a Demiguise Derby race and stole the takings (FW).
demi = half, lesser amount + guise = an external form, typically concealing the true nature of something