The Collaborative International Dictionary of English v.0.48:

Mangabey \Man"ga*bey\, n. [So called by Buffon from Mangaby, in
   Madagascar, where he erroneously supposed them be native.]
   (Zool.)
   Any one of several African monkeys of the genus Cercocebus,
   as the sooty mangabey (Cercocebus fuliginosus), which is
   sooty black. [Also written mangaby.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

mangabey
    n 1: large agile arboreal monkey with long limbs and tail and
         white upper eyelids