Perhaps what some may fail to realize is that in a 'wild' herd situation, not all horses get along. And those individuals are essentially driven from the herd and told to go live somewhere else.
In a domesticated system, that's just not an option in a fenced area. And certainly we aren't talking about 'wild' horses, however that particular instinct still survives in the 'domesticated' horse.
Simply put, not all horses are meant to live together in one big happy herd.
I definitely agree with this.
I just recently moved to a new, smaller barn, and Zeus went from being turned out in a bigger herd (10-15 horses), to being with just three others. He had absolutely no problems in the big herd. He adjusted quickly to the first herd, established his place as leader, and that was that. He was the calm center of the geldings, no one messed with him, and he would put misbehaving horses in place, calmly and quickly. When we moved him to the new barn he was turned out alone for the first few days, met the other three over the fence, and everything was fine. We decided to first turn him out with their older gelding, Shadow, who had previously been the head of their small herd. They seemed fine at first, checking each other out, grazed a bit. The second the Shadow pinned his ears, Zeus pinned his back and kicked. That was that. The older gelding moved away and Zeus returned to calmly grazing. The BO and I were obviously watching them and ready to separate them if needed. Zeus had never had a problem with other horses. He's not aggressive, he's not overly defensive, he just wants to do his own thing and doesn't want any unrest. That one kick was the only incident, but a few minutes later, Shadow tried to assert his dominance again, and Zeus turned and pinned his ears, threatening to bite. Shadow absolutely FREAKED out, and tried to duck under the fence into the other pasture. He crashed into the fence, and when he couldn't get out paced the gate, calling to their other two horses. We let him out, and he was fine, just scared.
We both couldn't see any reason for him to suddenly become so afraid. Zeus neither chase him, or was overly aggressive in his responses. We guessed that since Shadow had never really been challenged it really shook him realizing that he was no longer in control. Shadow and Zeus are also stalled next to one another, and still, after three months, pin their ears at each other whenever they make eye contact. The problem was sorted out, and Zeus is now turned out with their other gelding who is perfectly content to let Zeus be in charge. They graze next to one another, groom each other, and get along really well.
In short, Shadow and Zeus just clash. Shadow really is afraid of Zeus, who may be calm and only get after him when necessary, but is bigger, younger, and stronger, while Shadow is old and overly defensive. They just can't be turned out together, even in our huge pasture.
If anyone has more ideas as to why Shadow freaked, feel free to post them.
But I think it has a lot to do with the fact that Shadow is weak in his old age and knows it, and responds by being defensive.