tessiers,
Well, I guess if you would have availed yourself of the slaughterhouse as an "outlet" for an animal you can't feed, then you are ok with killing the horse, because that is what will happen to it. Once we're over the philosophical hurdle of what ultimately happens to the horse, to answer your question, you could shoot it and bury it on your (or someone else's) property. Sounds pretty crass to horse lovers and don't know what laws you'd be breaking in your area, but a neighbor of mine did just that a while ago.

The only difference from the slaughterhouse option is that you wouldn't profit from the horse's death. But it sounds like you're in "cost avoidance" mode at this juncture and not looking to make a buck.

At the barn where we keep our horse, the vet is called to do the euthanizing and someone comes to get the horse. Don't know what the cost of all that is.