So further thought on trailering and giving a horse treats to do it.
Asking a hard to load horse to come into the trailer for a treat, grain, hay, buddy etc. is extremely pointless. Now before you get your panties in a bunch listen to the explanation of how a horse feels about this situation.
Say you and your friends were at the grand canyon. Remember this reference from a few posts ago? Yeah well they said they would give you a one hundred dollars to jump off. You look down into the canyon and theres many, many feet of sharp rocks, ledges, trees and other things that will hurt you. You know you are going to die if you jump. But they protest. They say "We did it and we are alright." You look again, no way would you even be able to jump as far to miss the first ledge with large pointy rocks. You back away and refuse to go, knowing you would see the end of your days if you jump, despite the fact your friends said they were ok.
See where I am getting?
When you take a single carrot and ask a horse that is afraid to load in the trailer you are asking them to jump off the grand canyon for that money. They think a dark, closed in, scary, noisy, maybe windy box may end their lives. It would be like a wild horse walking into a cave. That cave might have bears or lions in it and if they go in there most likely isn't an exit directly in front of them. You can use food in ways such as every time the horse puts a foot closer he is rewarded. I've used that for my pony after the fact he would load normally. I wanted him to have a reason to want to load other than me asking him. You need to have that trust there as your parachute. You need to show your horse that trailering isn't scary and it's actually better to hop on the trailer than balk. Remember it takes two to fight.
So after some time on the side of the canyon and you decided you wouldn't jump your friends say "Oh yeah, by the way you get a parachute" So you decide to jump after all and got that $100.