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Well, from my experience it is eventually alright. I had a quarter horse gelding that was blind in one eye, completely. He was my barrel racing horse, so it was hard to break him of his habits on the "bad" eye (he was home broke with some help) and he ended up fine. The trick we found was to continue what we were doing on that side. We basically got him used to what we were doing more on that side than the other side. We tried not to treat him differently, or specially on that side, just worked that side more.. with stuff like brushing and turning towards the "bad" side. If you want to just lunge him for now, work on turning him into tight circles, you have to take charge over him. Let him know that he has to move to that side, but you're not going to hurt him.
Do the same thing while riding him when he's broke. A trick we used to get our horses to bend more when I was still barrel racing was to unhook the reins on the bridle and pull them to one side and tie them to the horn. It sounds stupid, but it will get him to turn into that side better. His problem could been that because he realized and his last owner realized he couldn't see well out of that eye, he baby-ed it. Now he's just used to not using it. With some persistence he should be able to turn as good as my quarter horse.. he can do a barrel pattern on both sides, without any problems..
I hope I helped alittle, if not, I give you more direct advice.
Source(s):
14 years of riding. (Barrels, 3-Day eventing, Stadium, Thoroughbred Racing)