Training abused and/or abandoned horses

You have gotten great advice here. Treat them like any other horse. Knowing their mental limits is a must. And having good timing of taking off pressure is critical.
I know a mare who was seriously abused for a length of time, while tied up in stocks. Horses are "flight" animals, only when necessary will they "fight." This mare learned that she couldn't run away from danger, so humans essentially taught her to "fight." The woman who rescued her thought it would be best to gain this mares trust by being her "friend." Don't get me wrong, this woman is great with horses and very knowledgable. But this is a mistake a lot of people seem to make with abused horses. The mare is still aggressive at times, but some of that is because her new owner wasn't sure of when to punish an abused horse, for fear of losing the horses trust. So now, she will still bite and kick when you saddle her or do things she doesn't like. But she no longer tries to run people down and attack them. Treat these horses like any other horse, they don't need any special treatment. Just good horsemanship.

I leased a horse that had been abused in its past. My trainer advised me that babying previously abused horses is counterproductive- above all they need consistency. They need to know what the rules are, that they must follow those rules, and that you won't punish them arbitrarily. I'd venture that most people who physically abuse horses aren't very clear or consistent in applying "the rules" and the horse has no idea why he's being punished.