Etymologyridge + -ling, presumably from the supposition the remaining testicle(s) remain(s) near the back
A ridgling or rig is a male animal with an undescended testicle. An undescended testicle is not a serious or life-threatening condition, though it may cause the animal discomfort at times. This condition can be corrected by either surgery to place the testicle in the correct position or by castration to remove the testicle altogether.
Horses that are ridglings are usually gelded, though owners of a prized animal may opt for surgery to preserve its value as a breeding stallion. An example of a ridgling that was gelded was the Kentucky Derby winner Funny Cide. While the horse was gelded because the undescended testicle caused it noticeable discomfort and its manners also improved after castration, its success in horse racing has led to considerable discussion over whether it would have been possible to opt for surgically correcting the condition instead.
ridgeling in German: Klopphengst
ridgeling in Dutch: Klophengst