The Collaborative International Dictionary of English v.0.48:

Rhesus \Rhe"sus\, n. [L. Rhesus, a proper name, Gr. ???.]
   (Zool.)
   A monkey; the bhunder.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rhesus
    n 1: of southern Asia; used in medical research [syn: rhesus,
         rhesus monkey, Macaca mulatta]