The Problem: Your horse refuses to load in the trailer.
Understanding Why: Trailers make horses feel trapped and claustrophobic. As prey animals, with a flight or fight response, horses prefer to be in open spaces where they can easily see predators approaching them and then be able to make a quick getaway. Trailers are also scary objects that move and make a noise.
The Solution: Get your horse comfortable around the trailer. If he’s not comfortable around the trailer, he’s not going to want to get inside of it. Start by asking him to move his feet with energy around all three sides of the trailer.
When the horse is comfortable moving around the trailer, lower the ramp and send him back and forth across it. Sending the horse across the ramp will help him get used to the noise the trailer will make when he steps up onto it.
When he’s calmly walking back and forth across the ramp of the trailer, use Approach and Retreat to ask him to take one step inside the trailer. Just before he gets nervous or plants his feet and refuses to move, back him out. Then approach the trailer again.
When he's comfortable taking one step inside the trailer, ask him to take two steps, and then back him out. Continue using Approach and Retreat until his whole body is in the trailer. Then let him rest and relax in the trailer as a reward.