Breeding ponies are can be of either male or female and the sex can be simulated or real. The difference between breeding ponies and pleasure ponies is breeding ponies have sex with other ponies as opposed to their owners/trainers (at least during a scene). Two (or more) breeding ponies may be bred in front of their owners/trainers and even other people.
The opportunity for incorporation veterinary play, especially reproductive exams, is quite large for breeding ponies. Moreover, preparations for breeding also provide ample opportunities for scenes.