Gelding becoming more aggressive
I have owned my paint gelding for about 7-8 years. He ridden by 2 inexperience young boys (ages around 4 and 5), and was green broke when I bought him. He was an absolute sweetheart, and although very spooky, had very good ground manners, was very polite, and etc. The first night we had him home, he was (we think) bullied by an older gelding, and jumped the fence. He was calmly standing on the front lawn the next morning.
Fast forward about 3 years: I boarded him with a trainer, and came to ride him every night for 2 months during the winter. I was the only one riding him, and he got turned out alone between rides. I was inexperienced, but gentle, and very timid. He was very polite, but slower to learn. Still, he didn't do anything out of the ordinary. But one day he started to buck a bit. I was just trotting around the arena. I took the saddle off, and rode him bareback. He grew tense, and stomped his feet in the ground, like his back hurt him or something. We decided to turn him back out to pasture, thinking he might sort himself out (we hadn't heard of equine chiropractors at that time, and certainly didn't have any nearby).
Fast forward 2 years: Took him to a well-known trainer at a local exhibition. He got the buck out of him, and was very gentle while doing it. They really, really liked his disposition. Despite his original spookiness, he was very calm and curious around big machinery at the farm show. Took him home, and he began to buck again, but even worse. He'd put a rodeo bronc to shame. Haven't rode him for a year and a half. We wanted to sell him, but the market had bottomed out, and we didn't want him to be sold as dog food.
He is currently in a heard with 6 other horses - 3 geldings, and 3 mares. He gradually became the big ma-chees-mo, or alpha male, despite his being what we called a "boob". His position is pretty solid in the herd- he's the youngest of the 4 geldings, and strongest by far. But we noticed, after bringing him back from the trainer/show, that he started exhibiting almost stallion-like behaviors. He runs the other geldings around, snakes his head, chases everyone off from the big hay bales (and they are not starving by any means!), etc. But it seems as if he is getting more aggressive, if that's possible. We suspect that he just put another (older) gelding through the fence. Cut up the older gelding's feet horribly, had arterial bleeding, etc. Something that has never, ever happened. While we were tending to the injured gelding, the paint gelding tried chasing another gelding into the fence.
Does anyone know what could be causing this aggressive behavior? This paint gelding is such a sweetheart around children, dogs, cats, etc., and loves to be loved on. The vet said that he could be proud cut, but only if he displayed an erection while a mare was in heat. I've seen mares in heat around him, and have yet to see any kind of erection.
I just wish we could find the cause of his behavior. He's turning the whole herd sour.
Any info is appreciated! Thanks!