A ridgling, or rig, is a male animal with one or both testicles undescended, usually describing a ram, bull, or male horse, but cryptorchidism also can be an issue in dogs and cats. In horses, a ridgling is a male horse with at least one undescended testicle. Usually, both testicles in a horse have descended by the age of 16 months, but as many as 15% of all two- and three-year-old male horses may have a retained testicle. When a male horse thought to be a gelding exhibits aggressive or sexual behavior, cryptorchidism is suspected. Sometimes, a horse with an unknown medical history is actually a stallion with both testicles retained. An alternate definition of ridgling is a partially castrated horse. This can occur when a ridgling is gelded, but the retained testicle is not removed, resulting in an incomplete castration. Some notable racehorse stallions were ridglings, including Honor Code, A.P. Indy, and Slew o’ Gold.