I have found that some horses are just bullies - they will pound the poop out of another horse for some minor
offense, they have no sense of what an "appropriate" reprimand would be. To people they can be the love of the century, and to other horses...not nice. I am very protective of my mare when it comes to other horses, b/c she was either orphaned or weaned to early and w her lack of social skills, she can be plain annoying to her own kind w absolutely no ill intent...and she couldn't stand her ground if she had to. I make sure she does not encounter a "bully"...she may deserve correction, but not a beating.
So, my immediate action would be...separate them.