So I just gotta say that the other day I was doing chores and a potential new boarder was looking at the place. She told the owner she didn't like her horse to have any nicks on him so she wanted him with only one other gentle horse
Her horse is a draft cross YEARLING, not sure what she expects.....
Luckily that is a no go, dodged a bullet there.
But yes the way I understand this is the horses have been over the fence from eachother but if the two are together it results in a mark more than just working out the herd dynamics. Agressive horses exist. Some horses are instigators.