Why is someone called a guinea pig?

Answer 1

Because guinea pigs are believed to be widely used in laboratory experiments, when someone says you're a guinea pig they're telling you in a joking way that they (or someone else) is testing something on you, e.g., you're the first to taste something they cooked.

Answer 2

"Guinea pig" is a common name for a person who is being used to demonstrate something or being used as a test subject because guinea pigs are often used as these themselves: their gentle dispositions and intelligence levels make them an ideal test subject in animal testing or other practices.

Answer 3

somebody tests with you something that haven't been proved yet with anyone