Some monkeys use this as a fifth limb; it's prehensile, after all
Many New World monkeys in the family Atelidae, which includes howler 
... All members of the suborder have prehensile tails; however, the tails of some... [DOC] [TLE] Monkeys - Animal Facts and Information [PAR] Some monkeys are used in entertainment, by doing tricks, performing, etc.