As a horse owner and student of natural horsemanship, I would prefer that horses are in a more natural environment than a guide horse would experience.
I would opt for a guide dog if I needed this service.
A dog is a predator, a horse a prey animal. The instincts between these 2 species are opposite, making the horse less desirable than the dog as a guide.
I think the predator by nature, the dog, would be more comfortable in most situations encountered as a guide, than would a horse, whose basic nature is to protect itself from being eaten by predators.
And BTW, no you cannot potty train a horse.
They ideally eat in the trickle method, grazing is best. But if pasture is not available, many small meals of good hay and clean water during the day and evening is the best way to simulate a natural grazing habit.
This trickle eating, small bits at a time, keeps the gut working properly, and keeps a frequent and steady production of fertilizer exiting the rear of the horse.
A horse goes when it wants to, #1 or #2, look out here it comes....
The dog, being a predator, has larger meals, fewer times a day and can be trained to wait until it is outside to “go”. Horses are so different from dogs, they are opposites, predator v. prey.