I hear this problem a lot......For some reason it always seems to be ont he left lead, and when i ehar it I tihnk "Oh no, here we go again...."
I know you seem to have already ruled out the pain thing, but I'm jsut gonna open another window for you here. I already posted thi son another thread.
A while back we bought a ten year old reining horse gelding in hopes of using him as a lesson horse for my trainer's kid. At the time she had just put down the pony her daughter was riding and since I needed a new reiner and her kids needed a lesson horse, we split the costs and kept him at her training barn.
Well, after the first ride I noticed he would not pick up the left lead, he would not slide on his left lead, he would not do a flying change, and god forbid he turned around to his left.
Well, in short, even though he would lope to the left on his lunge line or on his own, when you were in the saddle.....nothing. Well, thank god I am on a first name basis with my vet. We were at a show together and I happened to mention it. He insisted I bring him in for an x ray. I did so, nd it turned out he had a bone spur in his left hip that was invisible to a normal vet check (no x ray) and there was no heat in the area or anything to tell me he was being anything but stubborn.
Just food for thought....I would not have thought about it, because he never showed signs of pain. Just irritability, ad I thought it was because the lady who had him before us (He reined until he was five, then sold for another five years to an ignorant gal who didn't know anything) and I thought she had just ruined him....I mean, you should've seen some of the horses she had. All had freaky habits like that. She was a total nutcase.
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.