Common carp have several different types of mutations that include leather carp (no scales) mirror carp (large spotty scales) and several others. They actually have a rather high mutation rate and new forms of mutations are continually being formed while others are becoming even more common like mirror carp. In some areas I believe they are close to 10% of all common carp. I've personally shot 1 leather and one mirror. Neither were full mutations but it was present.