The Collaborative International Dictionary of English v.0.48:

Guinea pig \Guin"ea pig`\ [Prob. a mistake for Guiana pig.]
   1. (Zool.) A small Brazilian rodent (Cavia porcellus or
      Cavia cobaya), about seven inches in length and usually
      of a white color, with spots of orange and black. Called
      also cavy. It is the domesticated form of the wild cavy,
      often kept as a pet and used commonly as an experimental
      animal in laboratory research.
      [Webster 1913 Suppl. +PJC]

   2. Hence, [figuratively]: Any animal or person used in an
      experiment; -- also applied to people who are unwillingly
      or unknowingly subjected by authorities to policies or
      procedures which might cause bodily or mental harm.
      [PJC]

   3. A contemptuous sobriquet. [Obs.] --Smollett
      [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

guinea pig
    n 1: a person who is subjected to experimental or other
         observational procedures; someone who is an object of
         investigation; "the subjects for this investigation were
         selected randomly"; "the cases that we studied were drawn
         from two different communities" [syn: subject, case,
         guinea pig]
    2: stout-bodied nearly tailless domesticated cavy; often kept as
       a pet and widely used in research [syn: guinea pig, Cavia
       cobaya]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "guinea pig":
   Cape polecat, ape, bar, bear, cavy, chimp, chimpanzee, coon,
   ferret, foumart, glutton, groundhog, hedgehog, laboratory animal,
   monk, monkey, mousehound, opossum, patient, polecat, porcupine,
   possum, prairie dog, quill pig, raccoon, skunk, subject, testee,
   weasel, whistle-pig, wolverine, woodchuck, zoril