Why is a horse trailer "high angle"? Well, most roads are HIGHer in the center and lower on the shoulder to allow rain to run off. So, if you have a horse trailer where the horse faces exactly forward, he's standing with his left feet HIGHer than his right (in the U.S.). After a long trailer ride, that can really wear down the horse.
So, most horse trailers made today are called "slant loads". The horses stand at a 45 degree ANGLE, which is much easier on their bodies. Here's my trailer, just before we did a ride today. The bar you see separates the two horses at the ANGLE that you see here.
Not exactly related to today's topic, but most animals recognize that a horse is NOT a predator, and they let my horse carry me much closer than if I were walking. This Egret let us get within just a few feet today before flying off. I just wanted to throw this in. :) . But, as I look at it, the bird IS holding his head HIGH and at an ANGLE to this trailer's leading lines. It's a stretch, I know.