The “long, fine, silky and silvery” pelt of a Demiguise is valuable for making invisibility cloaks (FB).
- The Demiguise resembles a "graceful ape" and lives in Asia. It's ability to become invisible makes it understandably hard to find (FB).
- There are three usual ways to create an invisibility cloak: weaving it out of Demiguise hair, using a Bedazzling Hex, or using a Disillusionment Charm (DH21).
- Demiguise-woven invisibility cloaks will eventually age and turn opaque (DH21).
- Invisibility is the reason the Demiguise is used as a rune for zero (JKR).
Harry's cloak was not woven from Demiguise Hair, according to Xeno Lovegood, or it would not have remained magical for so many years, through so many generations of the Potter family. It was made from some other substance as one of the Deathly Hallows, supposedly by Death himself - more like Ignotus Peverell - but we are never told the type of fabric or charm that was used.