Nothing. Receiving penetrative sex of any kind (oral sex too-- that's why 99% of the time, in bestiality porn involving blowing a horse, the cum is fake) from a stallion is pretty fucking dangerous.

Honestly, unless the horse is drugged or very restrained (which all of us here will agree is rape and a Very Bad Thing), nothing. It is a dangerous, vulnerable position, and not only could a stallion hurt someone intentionally if upset, there are also all sorts of potential accidents- being hit with hooves, having too much weight put on the human's body (depending on the position), and, infamously, even injuries from the horse's penis itself.

There's a little less danger with ponies and miniature horses because while they can still bite and kick, they only have one hundred pounds to back it up instead of nine hundred. Also their penises aren't as large, so people aren't as likely to die that way.

Miniature horses, brah.

your use of the word "Utilizing" I find slightly odd and saying you're "really into zoophilia" is also weird, like saying saying "I'm really into heterosexual sex", to answer the question, people that have sex with stallions are really putting themselves in a lot more danger than say mare, or at least it seems that way to me as I have never felt at risk behind my mare, just either don't mess around with horses unless you really know them and trust them and know what you're doing.