I began my serious riding career (as opposed to riding as a kid) with a man named Roy Traylor in Norco, Calif. Roy's "specialty" was fixing rank horses. He acquired that reputation in spite of being a top-notch fine harness trainer who switched to the hunter/jumper world. The reason he became known as someone who could "fix" bad horses was that he was willing to work with them and he was successful in that they were no longer dangerous while in his care.
I have to say right here that many of those horses proved to have physical problems that were masked by adrenaline when they were mad and acting out. As a result, my No. 1 priority in working with this kind of animal is to determine whether the bad behavior is due to pain or something else. I would say that at least half the time, the horse either is in pain or they associate being ridden with pain they have experienced in the past. If that pain is not due to unsoundness, I think it almost always is associated with big bits, bad hands and unbalanced riders. Getting a horse to give riders another chance at them is probably the hardest thing you have to accomplish with a sound, rank horse.
So if you find yourself in possession of one of these horses, what do you do? My best advice is to find another horse. Even if a professional gets the horse to a point they can ride it, chances are an amature will not be able to keep the horse going without a trainer's continued help.
This kind of horse can be very, very dangerous. Do not get involved with one of these guys thinking that if you love it, the horse will love you back and stop his bad behavior. Horses' brains do not work that way. Think of his brain like a computer. If you press Ctrl C, you are going to get the same result every time. So your task is going to be to change triggers/responses like: tighten cinch/buck; weight on your back/flip over; pull on the mouth/run away; foot in the stirrup/take off!; clippiers on the nose/strike; tied to a post/pull back; and the list goes on and on. I strongly suggest that if you want to own a horse for enjoyment and recreation, do not take on a horse with these problems; if you find yourself owning one, get rid of it - and for heaven's sake, DO NOT donate it to a handicapped riding program or some such inappropriate enviornment for a rank horse!
If you don't want to read any further, my bottom line is do not take on a problem horse unless (1) you have the skill (and I mean formal training and a lot of knowledge) to work with these guys; (2) you are athletic and resiliant enough to decrease the chances you will be injured; (3) you can afford an appropriate trainer for the horse and plan to leave it in training; or (4) you are willing to own the horse as a pet AND have a safe environment for him.