And just to note, the "20,000 horses!!!11!!1!" number sounds tremendous at first, but the US alone has 9,000,000+ horses. Somehow I think we can figure out what to do with 20,000 of those. That is .2% of the horse population in this country. The image being crafted a country overrun by neglected, feral horses because we can't send them off to be slaughtered is logically ridiculous and bordering on mathematically impossible.
I still think a bullet is the most humane. And there are numerous possibilities for disposal.