The Collaborative International Dictionary of English v.0.48:

guppy \guppy\ n. [Named after R. J. L. Guppy, who brought
   speciments to the British museum. --RHUD]
   A small freshwater fish of South America and West Indies
   (Lebistes reticulatus or Poecilia reticulata, originaly
   called Gerardinus guppyi), often kept in aquariums; also
   called rainbow fish.

   Syn: rainbow fish, Lebistes reticulatus.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

rainbow fish
    n 1: small freshwater fish of South America and the West Indies;
         often kept in aquariums [syn: guppy, rainbow fish,
         Lebistes reticulatus]