If a dog has been too rough, I've just stood up and let them know I don't like them getting that rough with me. I don't think it's usual for a dog at least to bite while mounting, since they take a hold with their paws. Equines then again can't get as good of a grip and they might bite more often than a dog. If a dog is a bit on the rough side, I'd think there was something behaviorally wrong with them tbh