In light horse breeds, testicles complete their migration into the scrotum any time from just prior to birth to a short time after foaling. Also, if both testicles have not descended into the scrotum within the first year of life, the horse is a cryptorchid. Due to the growth rate of young horses, it has been well documented that a light horse's testicles become too large to pass through the inguinal ring. There is little research related to miniature horses on this issue. Practitioners generally extrapolate the norms of light horses to miniature horses.
There is one article, written by J. E. Cox, Ph.D., M.R.C.V.S., Senior Lecturer, Division of Equine Studies, University of Liverpool, England, published in Current Trends in Equine Medicine 2 and Equine Reproduction. In his paper, Dr. Cox refers to the situation that occurs in miniatures as temporary inguinal retention. He states, "This occurs predominantly in ponies and is characterized by small testes, which are usually readily palpable in the anesthetized horse in dorsal recumbency (anesthetized and lying on his back). If these testes are not removed, they grow and descend into the scrotum usually by the time the animal becomes a three-year-old." Dr. Cox's conclusion would support the fact that the condition of your miniature stallion is within normal parameters. Many breeders of miniature horses indicate that testicles will be descended by the third year. If they are not descended by the end of the third year, then the stallion might be a crytochid.
There are different forms of cryptorchidism observed in light horses: under the skin in the inguinal area (high flanker); in the inguinal canal—there are two inguinal rings (the superficial and the deep), and the inguinal canal goes from the deep to the superficial inguinal ring; and in the abdomen. In one study of horses, the failure of the left and right testicles to descend into the scrotum occurs with nearly equal frequency. However, the left testicle is found in the abdomen in 75% of cryptorchid horses compared to 42% of right testicles. Bilateral cryptorchids, those with both undescended testicles, and monorchid horses, those with only one developed testicle, are uncommon. In bilateral cryptorchids, most of the time both testes are abdominal.
