The hooloovoo is a monster added in dNetHack. It is part of the U class, which in dNetHack indicates an "unknown abomination".
Hooloovoo are able to move over open ground and phase through solid rock; however, they are most comfortable on constructed walls, typically confining themselves to the edges of rooms.
The hooloovoo comes from the series The Hitchhiker's Guide to the Galaxy, where it is described as a hyper-intelligent shade of the color blue.
Little is said about hooloovoo, except that one participated in the construction of the starship Heart of Gold. This may be why the dNetHack hooloovoo is a tinker.