The Collaborative International Dictionary
Guinea pig \Guin"ea pig`\ [Prob. a mistake for Guiana pig.]
(Zo["o]l.) 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.
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.
A contemptuous sobriquet. [Obs.]
--Smollett
Cavy \Ca"vy\, n.; pl. Cavies. [NL. cavia, fr. Brazilian cabiai: cf. F. cabiai.] (Zo["o]l.) A rodent of the genera Cavia and Dolichotis, as the guinea pig ( Cavia cobaya). Cavies are natives of South America.
Water cavy (Zo["o]l.), The capybara.