There are so many factors contributing to body condition and while the feed program is the primary one, other issues can cause weight loss. I read that you recently de wormed her, but did you run a fecal test to identify what parasites she had? Not all wormers kill all parasites so it's possible she still has worms. A big belly and skinny top line can indicate heavy worm load or in youngsters, not enough protein in the diet. While unsatisfactory body condition usually indicates a lack of something, there are other factors leading to poor body condition. Have her teeth been checked/ floated? How much do you feed her now and what was she eating before the mini pony? I agree that what she is eating now does not appear adequate. Personally, I like mini pony for my easy keepers, in my experience people dislike a feed because they are not feeding it right or not feeding a horse it is designed for. It is a maintenance feed for easy keepers, NOT a breeding animal, senior or performance feed. An easy keeper or over weight pet pony has VERY different needs than a growing youngster, lactating broodmare or show jumper in heavy work. Just my opinion. I used to manage a feed dealership and often dealt with clients who had issues finding the right feed as there is no "one size fits all" ration. Senior feed is often used to put weight on thin horses and many seniors have weight issues. But perhaps your mare would benefit from a junior horse feed designed for growing bodies as 5 is still young. Hay forage is so important to the horse's diet as it makes up the majority of their diet. Alfalfa sounds like it would benefit your mare greatly. If you cannot find it (we had a bad hay season last year and everyone is short on hay this year) you can add alfalfa pellet or cube to the feed ration with quality grass hay. Research has shown that many horses benefit greatly from high-fat diets. You may find success adding stabilized rice bran or a supplement containing it such as Athlete or sunshine pellets (not sure what is available in your area). There are weight gain supplements and builders to add to feed for hard keepers, but in my opinion, quality feed and hay is a better investment. Some horses have difficulty processing feed and fully benefit from their feed and pro and pre biotics can help. I like source micro nutrients supplements and pro bios. I agree that your feed program needs reevaluating but I advise checking her teeth and parasite load. Remember to make feed adjustments slowly so as not to cause colic. An evaluation from a vet or nutritionist may help.