Guinea pigs were the small animals Andrew Ketterley used in his experiments with the Magic Rings. In his various trials, some of the guinea pigs died, and some exploded. Finally, one guinea pig was successfully transported from our world to the Wood Between the Worlds using a yellow ring. Digory noticed it there when he arrived to bring Polly back to Earth. The guinea pig seemed perfectly happy, so rather than return it to the cruel hands of his uncle, Digory decided to leave it in the Wood. It can be assumed that the guinea pig remained there, as the Wood's only known permanent inhabitant. It is unknown whether there were guinea pigs or their talking counterparts in Narnia.