I'm no professional trainer, but I do know of several horses who have been labeled "bullheads" or similar. One in particular I worked with on trailering. I know the owners pretty well, and they LOVE this gelding. In one summer he went from pasture puff (in almost the most extreme sense of the phrase) to participating in one of the bigger local shows in my area, worked with and shown by a local trainer. The horse has had loading issues in the past (never anything physically traumatic, but he has been slightly sedated once to get into a trailer), but he had been loading like a gem at home, going short distances, unloaded, ridden, reloaded, and taken home, no problem. After a day of showing, though, he wanted nothing to do with the trailer. By the time I showed up, over an hour had passed since the show had ended, the trainer had gone home with her other horse, and this gelding was not on the trailer, the owners were frustrated and losing patience, and every wannabe horse whisperer on the show grounds was hovering around the trailer. I jumped out of the car and took the horse, and in 5 minutes he was on the trailer, nice as pie. Horse went home, and the owners tried it again, and after nearly 2 hours ended up putting him away only having achieved getting 2 feet on the trailer. I went on down to their place and messed with the gelding for a while, and got him loaded again with minimal problem.
The moral of the story is that this horse is NOT afraid, NOT angry, confused, or upset in any way. Ears comfortably forward, eyes soft, tail loose, blinking, trying to graze, etc. Getting on that trailer was JUST NOT HIS IDEA. The trick with him was to make it his idea. I led him as close as I could get him to the trailer, got him to look at it and stand quietly, but when he'd back away I would make that backing up my idea as well, if that makes sense (not "in his face" backing up, more like Showmanship type backing, calm, steady backwards movement). When he got harder to back, not as enthused about getting away, I immediately reapproached the trailer and allowed him to rest in it, no pushing back, no nagging forward, etc. Pretty soon, he got the idea that if he chose to go away, he was by extension choosing to work. Choosing to do as I asked and get on the trailer got him rest and relaxation.
I don't see that there is much physical force involved here. This particular horse has been proved to that he is stronger than a human, and if you try to force him physically (i.e., drag/shove his butt on that trailer) he WILL and HAS thrown his weight around, which I cannot hold, so I can't afford to start something I can't finish. All firm, gentle request, no pushing, no pulling, no food motivation, happy horse, happy owners. The funny thing is that many owners take it personally and think that it's because they were ten minutes late feeding this morning, or they were out of town and a neighbor did the last barn check before bed. IMHO, horses don't have an "I'll get them back" capacity, or a spite gland.
So, there's my novel for the day, lol.