I'm not sure why you'd ask a forum if you've got a trainer who can show you how he/she's dealing with it, for when you take horse home? Isn't the trainer making headway?
How interesting that another horse who did this in fact had ulcers! Certainly, you'd want to rule out any physical factors!
If it's a clear case of the horse dominating the prior owner (not a fear issue of any sort, but rather a deliberate, thought-out tactic to escape being ridden), then it's a matter of a balance between matching his refusal energy plus 4 ounces back at him, yet also motivating him to want to be ridden by making it as interesting as possible. The unmotivated/domineering horse needs time spent with the human to be interesting, or else reverting to balking in some form is inevitable. This type of horse is food-motivated, & I've seen them get interested with random treats (not every time), till you don't need them so much.
Last edited by Northern; 09-23-2011 at 06:51 PM.