The Collaborative International Dictionary of English v.0.48:

Guenon \Guenon"\, n. [F.] (Zool.)
   Any of several long-tailed arboreal African monkeys, of the
   genera Cercopithecus and Erythrocebus (formerly
   classified as Cercocebus), such as as the green monkey
   (Cercopithecus callitrichus) and grivet (Cercopithecus
   griseo-viridis).
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

guenon
    n 1: small slender African monkey having long hind limbs and
         tail and long hair around the face [syn: guenon, guenon
         monkey]