Actually, from what I've heard, it is more commonly used on females as there is less chance of permanent damage if the wooden pony is used incorrectly. If the dominants (or yourself) are not aware of the safety concerns regarding a wooden horse/pony, it may result in someone not being able to walk without pain for long periods of time.
Believe it or not, male testicles can much more easily be damaged than any parts of the female's genital area; which is why males are never left on wooden ponies for extended periods of time. (However, we're talking strictly BDSM... not medieval torture methods)
A safer method for males: position a violet wand or a similar device at the top of the pony, and adjust the length of the legs (or ceiling suspensions) so that the male can stand on their toes without touching the wand. If they are to lose their balance, they would have to go down to their toes and touch the wand. Make sure you are watching though, as violet wands are nasty if they remain in contact with someone for too long. (Especially on someone's... balls.
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