According to most sources, there are two sizes of unthunk: regular sized (standing about 7 feet tall) and the giant-sized (anywhere from ten to twelve feet tall). Whether they are separate species, or simply juvenile and adult versions of the same species, is not known.
The unthunk is a solitary creature that usually dwells in the desolate barrenlands known as the Screaming Wastes. Mostly living things cannot long tolerate the terrifying howls and moans heard at night in the Wastes (and for which they are named), but the unthunk doesn't seem to be perturbed by these sounds, leading some naturalists to speculate that the unthunk has no organs of hearing. It does, however, have a very large mouth, and has been observed ingesting everything from cacti to livestock.
The unthunk is unpredictable, to say the least. It has been known on many occasions to shy away from humans, but there are also several verified cases of the creature stalking and attacking people. There are also a few reports of unthunk seen fighting in the service of Nightbane.
-- from Balthazar Budd's Flora and Fauna of Wildernesse