I agree with Lucy Balmer. It usually is just one horse charging at another but it generally isn’t anything worse than that. Also just be careful about your horse getting herd bound. My horse is in a paddock by herself (next to other horses) and she gets very herd bound. I can’t imagine what it would be like if she was in their paddock!
Although it may not look nice for some to see their horses fighting, horses fight one another all the time in order to test to see who is the 'higher horse'. In a herd, horses will make their way up herd 'rankings', communicating with others through the only language they understand; body language.
They don't fight to intend to hurt one another, rather they fight to move the other horse's feet which ultimately results in victory. You see it all the time. When a horse approaches another, you'll often see this horse moving away from the approaching one to offer it space and to submit to the pressure that the lead horse is putting on them. They're social prey animals and work in groups, unlike predators who will fight in order to intimidate and punish.
Honestly, I'd just let them have a conversation and discuss who is the 'higher' horse between the two of them. Once that is established you will very rarely see them fighting. One of them will submit to the other eventually.
If you get involved and break it up you're not allowing the horses to communicate as horses should, therefore ending a conversation which has not yet come to a conclusion over which horse is the 'higher' horse between the two. As a result, they will continue to fight until the pecking order is established.
Bonded with any other horse until they rode together
Have you tried riding them at the same time? Like I know a horse who never