A ridgling or a rig is a horse with a retained testicle if the dropped testicle has been removed. This used to be common when abdominal surgery was all but unsurvivable by horses. (Nowadays, most ridglings have the retained testicle removed).
Ridglings were notorious for acting "studdy" and showing aggressive behavior, and one of the things people would do when buying a gelding was look to make sure there were two castration scars. (Bizarrely, I once knew a ridgling who was one of the quietest animals ever and spent years teaching children to ride - his owners just never told the parents).