People often wonder why animals that are cryptorchid need to be neutered. The reason is two-fold. Animals that are cryptorchid may still be fertile, especially if unilaterally (one-sided) cryptorchid. Cryptorchidism is a known heritable trait. This condition is more common in some breeds, but seen in all breeds and is passed on from father to (male) offspring.
In addition to the heritability aspect, the testicle (or testicles) that remain in the abdomen are more prone to cancer or torsion (twisting and cutting off blood supply).
I remember a case of a geriatric Irish Setter that had been neutered as a pup. Apparently, only the testicle in the scrotum had been removed during surgery. This dog was presented for difficulty defecating and urinating, with a large abdominal mass. A very large (12" long) testicle was taking over the abdomen! Thankfully, surgery went well, and he could live out his senior years comfortably.
When should my dog be neutered?
Opinions will vary, but if your dog or cat is still cryptorchid (one or both testicles) by 6 months of age, it is highly unlikely that the testicle(s) will ever drop, and the animal should be neutered to prevent testicular tumor formation and passing on this genetic trait to future generations. If the testicle is not readily palpable in the inguinal canal (groin area), abdominal exploration is necessary to find the retained testicle.
While many animals are neutered at young ages, it is my personal preference to wait until 6 months of age to handle the general anesthesia. Again, opinions about maturity vary as do anesthetic protocols used by your vet, so please consult with your veterinarian as to the best plan of action for your dog.