I had this happen with an animal once. I sold her to someone that said she would keep her and make her a part of their farm, well, they sold her months later to someone who then sold her at an auction.
It's not easy, and you will wrestle with it for a while, and you could certainly contact the people and ask what happened. it might put your ming at ease if they explain what's up and maybe they are working to find her a good home too,
but, you can make yourself crazy with guilt. YOU believed where she was going was ok and they would keep her, so you don't have to feel guilty about what they have done. Good luck, it's not easy saying goodbye to a horse, even if it's one that's too much.