You need to make sure your mare is healthy before you just try upping her food intake, or adding fat. There can be a serious problem going on that you may not know about. I'd call the vet and have them do a thorough exam of her first and foremost. A bloodtest may be a good idea to have done. As soon as you get a good note that there's nothing going on internally, then you need to definitely do something about her food intake.