In normal male canines, the testicles develop in the abdomen and then descend into the scrotum before or shortly after birth. In pups with a condition known as cryptorchidism, however, either one or both testicles fail to descend within two months after birth and are left underdeveloped and non-functional. The undescended testicle(s) may remain inside the abdominal cavity or else drop into the groin tissues outside of the scrotum. [...]
Cryptorchidism occurs when one or both of the testicles in a colt do not descend into the scrotum. This condition can occur in the male of any species, but it is often noted in specific lines of horses, so it is likely to have a genetic or inherited component. When a colt's testicle or testicles do not descend the colts is called a ridgling or rig, or may also be known as a high flanker if the testicle is located just above the scrotum under the skin.
In all male fetuses the testicles are first developed in the abdominal cavity, then at about the second week of development they move down through the inguinal canal, through two sets of inguinal rings, into the scrotum. The rings are muscular areas that help to move the testicles down into the scrotum. In some male fetuses, through hormonal, mechanical and developmental reasons either one or both of the testicles do not move into the correct location. [...]