It's possible you're thinking of a different animal, or maybe you saw a guinea pig with an injury or a deformity. If you're concerned about a guinea pig's toes, it's best to consult with a veterinarian.
Why do baby guinea pigs have an extra toe?
Baby guinea pigs don't have an extra toe! Guinea pigs have four toes on their hind feet and three toes on their front feet. This is true for both babies and adults.