My guess is that he is trying to protect you from the horses. Pit Bulls were once used in bull-baiting, and to bring down unruly cattle, and many game-bred Pit Bulls today hunt and catch wild pigs. It could definitely be a combination of confusion (because I'm sure he's never seen a horse before), drive, and the steadfast loyalty that the breed is known for. The fact that he doesn't go after them unless they're in motion or around you definitely supports the drive theory.
I would absolutely keep him leashed around the horses. Attacking them is VERY bad, and VERY dangerous, for him and for the horses. Having a trainer come in is a great idea... assuming that the trainer is positive reinforcement only. Hurting or bullying him into accepting the horses could likely create a lot of other problems on top of the current one. It is possible that all he needs is some solid Obedience training (a rock-solid stay, recall, and "leave it" is important) and lots & lots of desensitizing and counter-conditioning.
What do you mean by "inside" dog? If that means the dog isn't left alone outside unsupervised, yes, that's a very good idea. My dogs are "inside" dogs and they are nowhere near miserable. Dogs want to be with their people, so it's natural that he should be inside when you are, and out when you're out and he can be kept safe/leashed/supervised.
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