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From 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]