My horse is the same way just keep him occupied. Keep him walking and do a lot of circles.
My barrel pony is just like that. He does lots better when we walk him and have him with another calmer horse.
Maybe try taking him to other stables as much as possible? My horse used to get very nervous but we spent about 1 year going away as much as possible (not only shows), so now he doesn't really care.
Rub the part of his head right where his mane lies, it is a pressure point that relaxes most if not all horses.you can also give him some natural herbs and talk in a soothing voice.
If there is a round pen open at the show lunge him a little. Also if you're on him rub his mane. It's something a mother use to do to its foal to keep them calm. I had a gelding that got anxiety being at a show and I always thought lunging him and rubbing his mane helped.
If his issue is tacking take him to a show that your not riding in and just tack him and let him stand and calm down, then untack wait a bit and redo. He is probably just anticipating the work and is excited if hes fine once your on.
I'll often leave my guy saddled standing at the trailer with a hay bale in front of him and he's happy as a clam, especially if im there with out a groom.
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If you are able to bring him to the property before the show several times to school it should bring up his confidence in that area before the show and therefore improve his confidence going to new places and showing.
One of the biggest things is to walk them around at the show as much as possible as soon as you get off the trailer. The movement will be destructing, and they'll get to see what's going on. Also have a hay net near by for them to eat from, and lots of water because nervous horses can become very dehydrated.
Perfect prep honestly works wonders
this year my whole goal was to get my tb mare cool and relaxed through dressage shows ..and my preparation routine and my own calmness helped here tremendously.stay away from nervous people etc...they feel our energy so much ..we did not even know about...good luck !
He's perfect once I'm on him, I just have to calm him down while tacking. I have a routine down with him.
This is super common and the best thing is to just expose them to it as much as possible. Start at small schooling shows and work up. try and create a relaxed and layer back environment, your horse can only be as relaxed as you are