Except horses aren't just put down. Many US states have banned horse slaughter, so the horses are often put on double decker trailers (illegal) and shipped to Canada or Mexico to be made into roasts.
YouTube - Horse Slaughter in Canada! (warning!!!)
If one cares to one can google double decker trailer and horse and click images. Not pretty. Horses are often shoved to many in as well as it being to low. Horses have been trampled to death on the ride, ended up with parts of their faces missing, broken bones...
Its not the same as being put down in a shelter. Its far far far worse than what happens to unwanted dogs. I am not against the eating of horses, I think cows are tastey. I am against the inhumane transportation and slaughter practices that happen to once loved companions (it would be just as bad if they weren't somone's loved pet, it just seems worse to me that a horse that has known such care and love to then be so abused and alone because people are to cheap, or have no easy method to have a horse pts and the body removed)