Mare vagina is absolutely the softest, smoothest, slickest location on the planet. They even have the ability to pull it open from end to end. So stallion penis is incredibly sensitive. Stallions are frequently put off by the feeling of tail hair dragging across their penis which is one of the reasons you often see mare tails cut or bound.
So when you see someone frantically jerking on a stallion or gnawing on him it's a safe bet that the horse is drugged into near unconsciousness. Otherwise he would be out of there in a flash. The drugs also have the effect of forcing the relaxation of his retractor muscle resulting in the dropping of his limp penis.
Contrary to popular belief, horses do not have a sheath. If you look at a male horse with his penis retracted you will see 2 lumps on the bottom edge of the hole in his belly and some loose skin behind it/between it and his balls. That's his teats in pretty much the same place as a mare's. When he contracts his retractor muscle the smooth section of his penis between his glans and medial fold is drawn up into his belly through that hole. The skin between his medial fold and belly is not attached to the erectile tissue inside it. That skin turns inside out around the section of his penis in front of the medial fold.
Why does this matter, you ask. Because if a stallion is dropping to piss or just hang out it will be by relaxing that retractor muscle and the loose skin closer to his belly will come out first and protect the more sensitive glans and front end of his penis. If a stallion is contemplating sex and having an erection, the erectile tissue will push his glans and the front section of his penis out first. In fact, the final fold of that loose skin won't come out of his belly until he thrusts. Knowing this gives you a reliable barometer of the horse's true mood.