they have a lock jaw mechanism
Though their jaws are definitely more strong than that of some other breeds, the 'locking jaw' is an old wives tale. They don't actually lock. Just wanted to throw it in there. Many people keep saying they want to ban and euthanize all bully breeds and this is one of the reasons they use to scare the public, telling them that their jaws 'lock' and if they bite you there is no way to get them off.
Anyway, it sounds like she needs some very serious remedial training. Keeping her totally away from the horses while you re-establish your role as leader is essential. I was going to post that if you don't do something quickly she's likely to be kicked or stomped by a horse and she could quite possibly be killed. Too late there I suppose. Luckily it didn't turn out as bad as it could have. But the chances that she's 'learned a lesson' from it are pretty much slim to none.
Again, start with the basics with her. Sit, down, stay, etc. Then keep her in a confined area and do running with a quick down. She must learn that as soon as you tell her down she must immediately drop to the ground. This is not only for the safety of the horses if she heads toward them, but in case she takes off after something else that may be headed toward a road with vehicles or something. It's in her best interest to mind you every single time, and you must be the one to enforce that.
Once she's master the down technique from a run, put her on a long training lead and take her to where the horses are. If she shows interest, give her a very quick correction and a down immediately. You need to make her understand that they are your horses and you will not tolerate her aggression to them. She also needs to learn that you are her boss
, not her playmate, or worse her property to take care of and defend.
If you can't get her broken from this behavior, it might be best for everyone involved to re-home her.