It's not a common expression. Guinea pigs are often used as test subjects in science experiments and we can use the term "guinea pig" to mean someone who is being used in a metaphorical experiment. For example, if you try a new recipe and serve it for the first time at a party, your guests are your guinea pigs. You're testing the recipe on them to see if it's good. If someone says "you've got the wrong guinea pig," it means they're unwilling to be part of your experiment.