Stiff horse on one rein

Hello. I would recommend having your horse checked by a vet to rule out pain. If everything looks fine, it sounds as though your horse needs to work on straightness and balance. Here are a few tips.

Start from the ground: Check that the horse tracks straight by leading him on a straight line, for example along the quarter line. Have someone else lead so that you can watch whether he moves evenly with both hind legs, or whether he pushes a shoulder out.

Use poles: Lay out poles to walk or ride through to help with straightness. Work at a calm pace and make halt-to-walk transitions to focus on keeping the horse straight.

When you ride: Make sure you sit centrally over the horse and ride with a longer rein. Pick up canter on a straight line and come back to trot or walk through the turns, finding the balance there first. Avoid pulling too much on the inside hand, which only creates resistance.

Strength and suppleness: Stretch the neck to both sides using a carrot or another reward to improve mobility.

As for the bit, it may be worth trying a milder one that sits steadily in the mouth, ideally in consultation with a vet to find something that works well for a horse with an overbite. Good luck.