A "guinea pig" is a kind of small, furry animal. At one time, scientists used guinea pigs for laboratory experiments. Today, we use the term "guinea pig" for a person who gets used to test something, like a new drug, product, or process:

He's been developing a new weightlifting routine and using me as a guinea pig. The results have actually been amazing.

Once in college I signed up to be a guinea pig for this medical test. They paid me a couple thousand dollars to take this drug that was supposed to help me quit smoking.

A person can "be" a guinea pig, or you can "use" someone else as a guinea pig.