My arab mare used to be owned by a woman who had 5 horses. To save time she used to feed them all their meals in the field, so there used to be plenty of squabbles over dinner. When she came to me 7 years ago, she was the grumpiest, snappiest horse I have ever known. Most people know a horse that MIGHT kick if you get too close, few people know one that will definitely kick if another horse so much as looks at it! Ironically, she was generally okay when out in the field - it was when she was being ridden, led or tied up near other horses that problems started. It seemed to me that she had a problem with situations in which she might become trapped. However, there was one horse, a young arab gelding, that she used to hate with a passion. One time, his owner turned him out and she literally chased him back out of the field!
There was no real way of dealing with this issue tbh. The yard owner began doing separate grazing for mares and geldings (not because of my horse) so she didn't have to see the gelding anymore, and I was always just very cautious with her around other horses. Anyway, a year ago I moved to a much smaller yard where the grazing is mixed, all the horses are fed at the same time in the winter by the yard owner and are also brought in and turned out by her. Well, my horse changed within several months. She would still kick sometimes if other horses got too close out hacking, but generally she was much more relaxed and better with other horses - I even started riding side by side with other people. I assumed it was because she knew them all quite well - in the summer there would only be 5 of them sharing half a hillside. Now, she is out on loan at another small yard and she seems to be equally relaxed.
So, I'm not sure how to help other than ask whether there are any external factors that could be contributing to your horse's behaviour. I think, having been in an environment where she had to fight for her food, my mare felt threatened by the presence of so many other horses all the time, and is therefore suited to a smaller, 'safer' herd.