I agree with Walkamile. I know at least one person local to me (very responsible horsewoman) who found a good home for the old horse of hers that is now packing little girl around, and horse is much more happy there being "in use". So situations can be different. With that being said I do think the owner is responsible for the old horse in 1st place, and if nothing works out I'd much rather put the horse down.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."