Ugh, OP, I’m so sorry about this.
I had a roly poly 15.2 Appendix gelding that I raised from birth many years ago. I broke and trained him, showed him, everything. He was ALWAYS the picture of health. Round (but not obese), gorgeous hair coat, just perfectly healthy. Never a sick day in the ten years that I owned him.
I wound up giving him to someone that I know takes good care of horses. He would be ridden more, jumped, used for lessons, etc. I figured it was a perfect place for him.
Less than a year later, I saw his new owner at a big show. Before I ever laid eyes on the horse, she came to me and started apologizing and explaining that they were doing everything they could to figure out what was wrong with him, etc. Then I saw him. If she hadn’t pointed him out, I would have never recognized him. His hair coat was scraggly and and rough, he was rail thin, he looked…sad. He had always been a bright-eyed fella, and very interested in his surroundings. The horse I saw before me didn’t give a crap about anything. I was heart-broken. I had raised him. He was my baby.
Maybe six months later I got a text from her that they had to put him down. He’d been colicking a lot and they just couldn’t seem to figure out what was wrong with him.
I truly believe he was stressed out (he was a sensitive “one person” kind of horse) and probably eaten up with ulcers. Being used as a kid’s lesson horse just didn’t suit him well. He was a good boy, but he wasn’t much for noise and chaos.
Fast-forward to just over a year ago. I moved my current gelding to a boarding barn after selling my place. Things were great at first, but slowly he started to deteriorate. Lost weight, top line looked horrible, hair coat looked horrible, he started getting spooky, started having diarrhea. It has taken the better part of a year to get this horse back to his fat, laid back, happy self. There were several things at play: different feed, different schedule, different place, being ridden more, the loss of his two life-long friends (who are buried on my old place).
I think some horses don’t “move” as well to new places or owners as others do. A horse that has had a lot of humans or been moved around a lot to boarding facilities or overnight shows, etc. probably handles it much better than horses like mine who were owned, raised, and trained by one person on one farm for the entire lives (with a few shows here and there, not many).
I hope together with animal control and maybe a local rescue group, you can achieve either getting the mare back or seeing her happier and healthier with her new person.