I agree of course that this is dangerous behavior for both of you and health checks are in order. It could be behavioral, but since your efforts to redirect him firmly are not really helping, my gut is saying there is something physical. Anything from a mild case of laminits, on up through the leg or a pressure point on his shoulder from the saddle, and god knows what else could be going on. For example if he developed a bruise on the shoulder point from the saddle, he might still be very tender even if you think you have placed the saddle far enough back. If he had bruised his foot and has a case of borderline laminitis in that hoof, turns in that direction will be painful, other direction might work. Check out his girth region really carefully, a wee girth gall can be extremely painful. Maybe he has a sore tooth or bars that only bugs him when going in one direction (lunging without the bit should isolate that possibility). Too much lunging towards his painful side could agravate whatever, so be careful about that.Out of curiosity did you test turns toward his non favorite side when it happens to be in the direction of the trail. You might also try a treeless saddle if you can find one, just to relieve pressure points and see if he'll move more normally. What seems like behavior issues can so often really be a physical thing that we just don't see. Then a chronic physical problem can quickly become a behavior problem, so even if you find a physical cause, you may have some retraining to do. Unfortunately horses can't talk, so the only way they have to tell us something is by their behavior. Basic rule, listen to and trust your horse. If you force him whatever the cause, he's going to rebel. So in retraining you want to back it up and almost start over with getting him accustom to the whole process, but each lesson bring him to the point before he starts to react, and just keep him cooperating. Every instance of rearing is just reinforcing whatever bee he has in his bonnet, and possibly triggering whatever fear he has of pain (if that is the cause). Use your common sense to stay safe, if you can't find an amicable way to keep him moving, then let someone else take the risks. That being said I personally would want to observe what someone else was doing with my horse as the wrong approach could make him a lot worse and then he is on a very short road to retirement.

Porte, my first instinct when I read your post was that your horse is five years old. This is such a common story with five year olds. They go along and then suddenly there is resistance. Every horse has their go-to evasion. Rearing is, in my opinion, the most dangerous.

Here are the health things I would check off if he were my horse:

1. Do you have an equine thermographer near you? If so, please have the saddle and horse re-checked. I do thermography and the camera finds saddle fit and physical issues that we humans miss. You would be shocked.

2. Also when was the last time the horse had his teeth floated - or even looked at? At five he could have something coming in or even falling out. Or he could have sharp points, particularly on the left side.

3. I would consider that the horse might have ulcers. It sounds like he was ridden quite hard from a very young age. Slowing down from barrel racing at three? How early did they start him??!!

You can discuss that with your vet. Ulcers are EXTREMELY common and painful. Even adjustments like not riding him on a completely empty stomach, checking his teeth, or changing his girth could help him.

Ok, let's assume he is phisically fine and the saddle fits...

Now, to the work. You've started shoulder-in and that is a collecting exercise. I don't know what else you are including in your program but five is often the age where the work demands are increased. For some reason there are general assumptions that a five year old should be doing this or that (anyway, that is another discussion). By the way, are you working with a coach? What do they say?

What I'm getting at is that your horse is probably feeling the extra work in his muscles and he is anticipating going to the work area and having to work. IF he checks out fine and IF the saddle checks out fine, do some good research and get someone with an excellent reputation (according to people with your same riding values) and let a professional help you get through this with your horse.

Five is a tricky age because the habits a horse sets down are usually theirs for life. You need to nip the rearing in the bud immediately, regardless of the cause, so the bottom line is have him thoroughly checked out physically and if he's fine, get help to address his work ethic.

p.s. He might prefer the western saddle over the English saddle because he knows the difference in the work he is asked to do depending on the saddle Horses are smart.

I think the fastest way to stop a horse from rearing is to bend

it's head around and that will prevent him from rearing.