A horse that rears is showing resistance. DO NOT WHACK THE HORSE BETWEEN THE EARS OR ANY OTHER OF THESE FOOLISH IDEAS. When a horse goes up you want the horse back down.. not over on its back or rearing higher.
Resistance can be because of holes in the education (if she was backing up to avoid things before.. the next segue is to rear.. seen that before and this is more of that). The horse has no clear idea of moving forward being the reward. Resistance can also come because the horse was an orphan and has no clear ideas about how a horse behaves or what boundaries are. All in all it adds up to the same thing... a horse that rears to evade a request from the rider or handler.
Golly knows why the horse is rearing. Get the horse gone to someone who understands horses and can train her or get rid of her before someone gets KILLED.
BTW someone who understands horses would not whack the horse over the head.. but would keep the horse moving and not give the horse time to think about backing up and rearing. They would not hold the horse back from going forward.. and let forward be the way out of things.
BEFORE this becomes and ingrained habit and the horse has to be put down, get this horse in professional hands. The more the horse rears, the more the habit becomes ingrained and the more likely the answer is to kill the horse before she kills someone.