Bogeymen
While not technically undead, verging more on anthropomorphic personifications, bogeymen are frequently lumped into this category. Bogeymen are manifestations of human fears, both childish and primal. They seem to be sustained by the fear of humans (belief has considerable power on Discworld), which gives them ample reason to keep to their traditional work of frightening people. When off-duty, quite a lot of bogeymen seem to frequent Biers, the unofficial bar for the undead in Ankh-Morpork.
Because bogeymen believe that they no longer exist if a child puts its head under the covers (although they only believe this because children do), bogeymen become extremely confused and begin to doubt their own existence if a blanket, pillow, or even a handkerchief is thrown over their own heads. Bogeymen do not have a fixed appearance, usually being able to manifest themselves as whatever the viewer fears most. Their true physical form is usually obscured by the frightening illusions they project. The only bogeyman revealed so far in the books resembled a small, wizened monkey.
The original bogeyman eventually became the Discworld Tooth Fairy.
Read more about this topic: Undead (Discworld)