How old is your horse? How much exercise does she get? I would recommend first feeding her more hay, preferably free feed of grass hay or oat hay or the like with some alfalfa thrown in in the morning and evening, which will generally help a lot with weight. Often times it is as simple as just giving them a bit more hay, and letting them free feed instead of just throwing a couple flakes in twice a day, and they gain weight in a healthier more natural manner. If you can't do that, or have tried and it hasn't helped, then I would recommend the Purina Omelene line of horse feeds, as it isn't horribly expensive, but it works well, has excellent ingredients, and was recommended to me by a friend who works in the nutrition department of a feed and tack store. I have a 4 year old Arabian, and on the Omelene 200, which is for foals and broodmares, she did great, never got any hotter than she already is, and I didn't break the bank feeding her. I would also recommend maybe adding Weight Builder which is a supplement made by Farnum, that worked wonders with my Thoroughbred mare, and I could just add it to the rice bran, and she gained and was able to stay at a constant weight while on it.