Let me butt in here!
First off, someone said the horse's head set when being ridden could be the same as his head set in the pasture. Well... In the pasture, the horse isn't carrying a person. I imagine carrying a person effects a horse's balance and therefore where it would be most advantageous to place his head.
Horses aren't born knowing how to carry people. People throw off the way a horse naturally moves. Maybe it's okay to teach a horse a better way, long term, to carry a person? A better way, I would think, would be with a rounded back and engaged hind quarters. How would I
do that? Teach the horse to carry its head lower, stretching the back. A horse with a rounded (or, rounder; gaits aren't performed with a back as round as a trot) back are inclined to engage their hind quarters.
I am an endurance rider. The effects of riding a horse for long periods in a hollow backed way include soreness (especially right behind the withers) in the short run, sometimes. But in the long run, it's worse. Back problems, improper muscling. The way a horse traveling incorrectly on the front end pulls itself wears on the front legs. Traveling this way is also very inefficient. Every good endurance rider I know concentrates on getting a horse moving properly
I don't see how "moving properly" for ANY horse includes an overly hollow back and pulling from the front legs. Conformation regardless --breed regardless, even-- it isn't a good way to move in the long run.
And for those who thinks it's uncomfortable for a gaited to travel with a low head set... My mare begs to differ. She loves traveling with her head level with her withers.
Why? Because she's learned it's easier
to gait long distances that way. Her "natural" head set was like a llama on crack. Just because she wanted
to do it didn't mean it was best. Sometimes, the horse isn't right. I believe this was one of those situations.
PS: I ride her in a rope halter.