The Collaborative International Dictionary of English v.0.48:

Mona \Mo"na\, n. [CF. Sp. & Pg. mona, fem. of mono a monkey,
   ape.] (Zool.)
   A small, handsome, long-tailed West American monkey
   (Cercopithecus mona). The body is dark olive, with a spot
   of white on the haunches.
   [1913 Webster]