An undescended testicle is a common occurence in the canine species. Such an animal is known as a "monorchid". A dog with only one testis descended into the scrotum cannot be entered into the conformation section of a dog show. He may, however, participate in obedience trials.
This conformation restriction is because the reproductive abilities of the monorchid are questionable. The condition does not inhibit either sexual desire or copulation ability. In certain breeds even neutering will not stop this biological instinct.