My daughter says to have the horse wormed and have the teeth floated. Might try the anti-sand treatment. One scoop every other day for a week. Helps the horse pass the dirt and sand it has in its stomach. The teeth need to be floated - ground down so she can eat properly.
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. Also, check her teeth. She may have a bad one, so she isn't eating or digesting well. Also, make sure her body is healthy and that she doesn't have lice or anything. That is usually passed from one horse to other, but its very hard on the very young, and older horses and cause them to drop weight. If you can't find any of these things, by looking at her, then for sure, call the vet, Good luck and I hope she is okay.
dear you should see veterinarian may she has problem with digestion he has to see her stool, may she have internal parasites also so you have first ti recognize the main reason.