Working with a previously abused horse

Retraining a horse is always a delicate dance.  One false step and you are back to square one or consigned to an oubliette of the horse's brain that's nearly impossible to extracate yourself from.  IE: the horse will equate you with the abuser and any dicipline will be equated with the abuse.  At the same time, you must exercise dicipline with the horse or risk disaster of some sort.  This sounds like a quandry but is really not.
 
Sometimes, it's best to return to square one and start over.  You haven't mentioned how old this horse is.  Since you've assigned the abusive behaviour with a second, or possibly third,&nbsprevious owner, I would guess the horse is over five years old.  If it's less than ten years old, you have a much better chance of starting over and working past the problems.  If older than ten, it's going to be exponentially harder.
 
Don't feel bad about what happened.  Actually it was a very fortuneate thing to find this out.  Now you know, it's time to work your horse past the situation.  You MUST keep at getting this horse going to the right.  Try riding the horse in the bullpen to the right.  Reward him profusely when the behaviour is desirable.  Be very judicious with  dicipline.  Judicious does not mean to neglect this important function though.  Many people make the serious mistake of just avoiding something like this and it only makes the horse worse.  Just choose your battles and when you have to do battle, try to do so in an assertive manner.  You have recognized the horse is not reprobate, just frightened.  So, you must teach him to face his fears and move past them.  Actually, that's the main focus of any training with a horse.  They're really a timid creature who recognizes that they have EAT ME printed in great big letters on both sides.  Hence, they're easily frightened and once frightened are difficult to deal with.  It's called a, 'fight or flight,' syndrome.  This is why some horses throw their riders and others run away with carriages.
 
So, to recap:  Your horse has a boogey Man in his background.  This individual did some seriously frightening things to the right.  In fact, the horse might even have been hurt, like hit by a car, ran into a post, or crashed into the arena rail.  They don't even have to have been seriously injured, as long as they're frightened.  So, your job is to make working to the right a safe, non-threatening thing.  Patience, unwearying patience, is your main training tool.  Along with a good dose of common sense and dogged determination.
 
Again, back to the basics.  If the horse won't longe to the right, try riding him in the bullpen to the right.  If he will accept being ridden to the right, then try putting his halter over the bridle (of course the reins on the outside), and have an assistant longe him with you on his back.  Be supportive, and try to let the assistant queue the horse.  You will reinforce the queues of the assistant.  Be careful when reversing to have the horse turn toward the assistant (the individual on the ground).  Elsewise the longe line will get hung up on you (the rider) and possibly unseat you.  Again, be assertive, supportive, and patient.  If this works, you can try longing the horse without a rider (with all the tack on), and see if there's an improvement, as there likely will be.
 
As to reversing:  I usually use the commands, "Come around," to the right and "Get around," to the left.  Thus, the horse associates the command 'come' to the right and 'get' to the left.  With the two of you working the horse this should be easy to teach him.  However, consistancy is essential  in this case.  I would at this time drop using the command 'Git' to encourage the horse forward (if this is your habit) as git and get will confuse him.
 
All the best.  There's nothing harder than undoing bad training.  Perseverance and patience are your best friends.

Thank you for such a thoughtful response! The horse is 13, so age could work against him, but fortunately, this is the first real setback that we've had in almost 5 months, and I completely agree with everything that you said. We're going back to the basics. I'm actually an animal behavior graduate student, so I'm familiar with "fight or flight."  This horse was a reiner, cutting horse, and barrel racer in his previous life, and he's trained to the hilt. His current owner, who has been working with me, assisted me yesterday to ensure that he went to the right and that we ended on a positive note (always important). I also discovered that this horse will get anxious if he's tied to the wall, especially if I leave the arena, which makes me wonder if that was a prior form of "punishment." He's 100% relaxed if I hold the lead or even ground tie him (doing this, he's as calm as my 30-year-old Arab mare, and his owner couldn't believe the transformation - he was almost asleep!)
 
I'm really excited to have found such a helpful and thoughtful forum - I'll definitely try your suggestions, and a few of them are in our current program. I'm very praise-heavy but never allow him to avoid something (like going to the right). I'm working him again this afternoon and will update.
 
thank you so much!

Update: Third straight day of succesful longing to the right. We completely went back to the basics and will continue to take it very slow for awhile. Thanks again for the advice!

I have seen some sad situations in my time with horses, but the one I am currently working with seems to be the worst yet.
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Earl Nunes

Anytime Telin, taking it slow might just be the ticket.
 
Okay Earl, drop the other shoe... What's the issue with this one, or issues?

I put in another ride with him on Friday - he was a perfect gentleman to the left (rocking horse lope) and picked up the correct lead the first two times going to the right, but then resisted (swung around to the left with a pretty fancy cow horse move). After he did this a few times and I couldn't correct him, his owner rode him for a bit and just kept gently but firmly bending him. I realize that all horses have a dominant side (left or right), but his behavioral changes make me think that when he resisted going to the right in the past that he was probably severely punished for it. Putting in another ride tomorrow - slow but steady is very rewarding.