You know, i married into the show horse business and i dont think people quite understand the calibur of a horse penis until they see one in person. i've seen a video of a guy take it up the ass from a horse, but they didnt plan on the length (easily a human arm length) to have such force and punch through his internal organs, and eventually die. I, too, watched in disturbed curiousity.
Horses are extremely powerful, and even moreso near orgasm, you should see them attack a fake mare for semen collection. The person who inseminates the horse wears a shoulder glove if that gives you an idea of the depth of a horse vagina. No human being should be able to handle this kind of depth, especially at the brute force a stud can deliver it.
I'll stick to well hung hairy human males, thank you very much!