From personal experience (as an involuntary - not to mention horrified - spectator, NOT a participant): a cat.
Yes, you read correctly - a cat. I was an EMT in Indiana in the late 1980s, and in between ambulance runs we worked in the emergency room. One night a gentleman (using the term loosely) walked into the emergency room wearing a trenchcoat. Now, as everyone who has worked an ER knows, trenchcoats are bad, especially in the summer. This trenchcoat was especially bad, since it was moving and hissing. Needless to say, we escorted the gentleman and his trenchcoat to the back rather quickly. As it turned out, he had attempted to have carnal knowledge of a cat, which apparently went ok until the cat had a sphincter spasm, trapping him inside it. The cat, being understandably quite upset and, btw, non-declawed, had made its irritation felt by stripping large amounts of skin off its assailant's torso and thighs. We finally had to call a veterinarian to give the cat a shot of muscle relaxant. Since the police officers were in the habit of stopping by the ER for a coffee refill on their rounds, and had been attracted by the commotion, we didn't have to worry about the proper disposition of our customer once he had been detached from the cat and bandaged. The cat went home with the vet.