Sweetie, the horse is old. Many don't live that long, and horses, like humans, often drop weight and get frailer as they get older. If you haven't brought in any new animals that would expose her to parasites, or anything, then it's probably not that. Is she getting all of her own feed, since she is with other horses? Make sure she isn't being pushed out by the other, younger, stronger horses that she is boarding with. If that's possible, try feeding her alone.