My horse is pasture mates with an extremely aggressive alpha mare. This horse is wonderful to work with (my kiddos learned to ride on her) but boy does she hate other horses. She's had to be kept alone for the past 20 years, done a ton of damage at boarding facilities because she didn't like being near other horses, scared a lot of boarding facility employees silly (we ran into one and she about had a heart attack when she heard that kiddos were riding her) because she learned that if you throw a giant kicking, screaming tantrum at meal time, a very nervous employee will feed you first so you'll stop. Our tack shed currently has 2 nice hoof prints through the door because something annoyed her.
When she was shipped out here, the shipper had to stop partway through the journey and move all the horses around because she was biting the one next to her constantly.
Strangely enough, she and my horse are best buds. We had the whole property divided up to keep the two apart originally but mostly there is no need for it. My horse is as quiet and laid back as you can get and ignores all the screaming, fence kicking and other attempts to rile her up. She's of the opinion that if you want to act like a stupid twit, that's fine and she'll go stand in another corner of the pasture or away from the fence that separates their runs. We feed them separately in their stalls but my horse has learned to open the gate in between the two, goes over and finishes up whatever the other horse leaves. She NEVER would have allowed any other horse to set foot in her stall, much less eat her leftovers. We've even found the two of them spending the night jammed in the same stall.
Took that horse 25 years to make ONE friend, she's going to be one handful to deal with next week since I am moving my horse to my trainer's place.