We have a little gelding where i'm at, he was "trained" in a fairly abusive manner and is slightly herd bound. When i first took my horse to the barn he used to charge at anyone who went here the field to get one of the other horses. he was the boss and his owner really didn't do much to fix it.
the barn owners received a huge draft horse as a boarder and put him in with this little pushy thing. little pushy horse went for the draft once, was put in his place with a nice big kick and was effectively demoted to the bottom of the herd.
i'm not saying putting this mare in with a very dominant horse would be effective, maybe, maybe not. i;m not a trainer i'm just saying it worked with this horse. Mind, now the draft has left he still sometimes gets aggressive but he backs down if you run at him waving your arms and yelling. he actually charged at me the other day with his teeth bared and i charged right back. (probably not wise, one day he'll probably just run me down)
we did have one horse at a barn i was at, a saddlebred show horse. he was so sweet one minute and trying to kill you the next. i was told he was bipolar but i mostly stayed out of his way until they sold him.