Cryptorchid, ridgling, and even rig are terms used to describe a stallion with at least one undescended testis. The condition is not unique to equids, but the horse is of specific importance as the retained testis fails to produce viable sperm, so fertility rates are affected. However, the testis is still capable of producing testosterone, so the animal will show stallion-like behavior. The cost of castrating a cryptorchid is significantly higher than standard castration, and retained testes are at a higher risk of developing malignant (cancerous) tumors.

In the normal stallion, the testes will have gradually descended from just ventral to (below) the kidneys, down through the inguinal canal, and into the scrotum, either in utero or during the first few weeks of life (see “Cryptorchid Anatomy” on page 43). The mechanisms that drive testis descent are not fully understood, although testosterone is thought to have a role. Occasionally, either one or both testes fail to descend, and such stallions are termed cryptorchids. A cryptorchid stallion might be further classified as inguinal (testis is in the inguinal canal), abdominal (testis is in the abdominal cavity), unilateral (one testis is not descended), or bilateral (both testes are undescended). Classification is important when considering management and treatment.

Inguinal cryptorchids, the most common type, are characterized by testes that have failed to descend beyond the inguinal canal, or occasionally lie just under the skin (called ectopic cryptorchids; see “Cryptorchid Anatomy”). Failure of inguinally retained testes to descend might be temporary or permanent. Temporary inguinal cryptorchidism is reported to be most common in ponies and miniature horses, and to mainly affect the right testis. If the condition is temporary, the testes will descend by three years of age. Failure to descend by this time indicates permanent cryptorchidism, a condition seen in al