Transgender Athletes

For many years there have been athletes that are in some ways transgender. These are thoroughbred race horses. An entire male horse is a "horse", a stud, or a ridgling, a male with one or both testicles undescended. A horse that has been castrated, whose testicles have been removed, is a gelding. A female horse younger than five years is a filly, older than that a mare. 

Horses that have personality issues are the one's that are gelded. These animals are sometimes too vicious to handle safely and dangerous to other horses. They may be so interested in mating that they are unable to concentrate on their work, winning races. Gelding usually has a tendency to pacify a horse and make it more tractable although if it's a success at racing it won't be able to reproduce more like itself. On the other hand, left entire it may not be able to show the talent that breeders want to see in a stud. Gelding is a tough decision to make with a well-bred horse.

Since gelding is always done to enable a horse to race, it must be entered in races. There are only two gender divisions in horse racing, horses and fillies and mares. Geldings, regardless of their testosterone level or self-image must compete with other horses. They are not allowed to enter races carded for fillies and mares. Fillies and mares are allowed to compete in races for males.

This has been the case for centuries and it seems likely to be the case for many more to come.