You should be able to #ride hostile monsters to climb atop them, which lets you have a much better chance at hitting it and possible higher damage when you hit it, but the monster also gets a much higher chance of hitting you and can try to throw you off.

There should be some common-sense limitations, like the monster should be big enough to climb on and capable of being climbed, and you should be in a form capable of climbing. Also possibly it should be harder, not easier, to hit the monster with certain types of objects, like cockatrice corpses.