Guinea pig: 1.Cavia porcellus, also known as Cavia cobaya, a plump rodent native to South America brought to Europe by the Spanish conquistadors. Often kept as pets and once very widely used a laboratory animal for research. 2.By extension, someone (or something) used as a subject of experimentation. In this sense, the term "guinea pig" may have negative connotations.
The name guinea pig is of unknown origin. In the pet trade, the term cavy is often used.