These are some of the most stoic and cooperative horses in existence.
When I was young I trained and successfully competed on high level eventing horses. I later found they had spectacular DJD (degenerative joint disease) and ringbone rumbling away in the background for years before it broke them down.
If your horse is refusing jumps, rushing, pulling rails, or generally becoming quite hysterical about jumping then he desperately needs some help finding his source of pain.