The animal Cavia porcellus, commonly kept as a pet or used in laboratories.
from Dutch marmot
I kept a guinea pig when I was a child.
Figurative use for a person or thing used as a subject in an experiment or trial.
He became a guinea pig in the clinical trial for the new drug.
Using humans as guinea pigs in drug trials raises ethical concerns.
He agreed to be the guinea pig for the new treatment.