When one or both testicles on the dog fail to descend. Testicles should descend fully by the time the puppy is 2 months old. If the testicle is retained, it is usually non-functional and can become cancerous if it is not medically treated. The treatment that is suggested is to neuter your dog. When the neutering takes place, a small incision will be made to remove the undescended testicle(s); the normal testicle, if any, is removed in the regular manner.