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]