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. [WordNet 1.5]
guppy, n. (pl. -ies) a freshwater fish, Poecilia reticulata, of the W. Indies and S. America, frequently kept in aquariums, and giving birth to live young.
Etymology
R. J. L. Guppy, 19th-c. Trinidad clergyman who sent the first specimen to the British Museum