You’ve gotten some great suggestions here! Let me add that many horses don’t like straight-load trailers because the inside is dark in the front. It might help if you can park so that the sun fills the inside and makes it look less Black Hole of Calcutta and more Good Morning, Sunshine. 🙂 A battery-operated light aimed at the front inside might help, but watch for shadows he casts as he moves in. Being afraid of his own shadow isn’t something you want to teach him. As for ramp vs step-up, most of my horses (including the foundered mare) preferred the step up to the ramp. They didn’t like the sound their hooves made on the ramp, and one nearly knocked us all over jumping over the ramp to get in. They are definitely individuals!
Horses In the Yard