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]