My horse came to be knowing how to bow, including with a rider on his back. I've never asked him to bow under saddle and he's never done it unexpectedly.
He also came knowing a cat stretch and how to give kisses. I'm not sure giving kisses is the best trick to train, as it teaches the horse to stick his face in your face. While I've never had a problem with my horse doing it too forcefully, he does do it without asking sometimes, and I could see it being scary to a little kid! I really like the cat stretch, though:
Garzo Performing His "Cat Stretch" - YouTube