guinea pig 
noun
1A domesticated tailless South American cavy, originally raised for food. It no longer occurs in the wild and is now typically kept as a pet or for laboratory research.
- Cavia porcellus, family Caviidae.
- A major debate was raised by the claim that ‘the guinea pig is not a rodent’.
- At school, our classroom had a small rodent zoo consisting of two rabbits, three hamsters, a litter of baby gerbils and a guinea pig.
1.1A person or thing used as a subject for experiment.Walker volunteered to act as a human guinea pig for these experiments, as did 400m runner Mark Richardson and one other athlete....
- The early study of carbon dioxide also gave rise to the expression to be a guinea pig, meaning to subject oneself to an experiment.
- The worst was when you became a guinea pig for a hazardous experiment, and not even the lightest punishment was endurable.