I have experience with both mares and stallions. I was young when I did it with mares. I had a dream first, that she pushed back to me and wanted it. Two weeks later that happened in real life. The other experiences were just more standing still, so I moved on from that.
A little after I was old enough to drink I bent over under a really nice quarter horse stud. He was just hanging his semi hard cock down as I groomed him, as stallions often do. He was fine with me handling his cock, so I put him in my butt. I had experience with large toys, of 10" girth, and had been fisted by a few people, but..... in retrospect, it's really good that he never flared. He slightly twitched and humped once. There were no mares near us in the barn. If he'd had the scent of mare (as studs new to horny humans need, I think) he'd have fucked my hole and flared.
I have a lot of skills with horses and have earned my living with them for short times in my life. I left that world because lots of trainers are assholes that are quick to anger, and my world view changed on some things.
Anyway, I often dream of getting a shetland pony stud and showing him that he can breed humans, in certain circumstances, and then introducing him to single women and couples (or trans males too) for getting him lots of pussy..... so to speak.
I currently own horses now, a gelding and mares, but they aren't to be intimate with people(my choice, and these not so gentle mavens would kick a mo fo through the fence). It would be tough to keep a stud pony on the same place. The big gelding could possibly be kept with a pony stud that would be put with him as a yearling, I'm guessing.
Anyway, I may have a mac daddy shetland in the future to lay some pipe. Not sure.....