Hi
Feeding any horse/pony can be hit & miss in finding the perfect meal. Over the years I've had scatty/fizzy to so laid back they seemed to be a zombie, scaredy cat to brave enough to take on the whole world. Very fat (barge) to very skinny.
When I got a new horse/pony or trying out a new feed stuff I found giving small feeds instead of say 2 large gave the horse/pony time to manage & adjust better. I know this is not always possible even I had to pay someone to give a feed. Some times I got it totally right, others no-one could, not even a vet or nutritionalist, I even had to change after the diet that had worked for ages stopped working.
I currently have a 5yr x-racehorse & had 14yr untill June. They both required a different diet in 2 or 3 hard feeds a day. If in all day vegtables (carrot swede apples etc) either whole in large chunks scattered in stable or if grazing a treat but not every week.
My 5yr old is currently on Allen&page Fast fibre, Top Spec High Fibre cubes & chaff with supplements of spearmint & a probiotic, hay & grazing, a new thing for him, took 1-2 weeks for him to fully understand. Which for him is enough & has gained weight since I got him in Oct last year. I found mixes, (barley most) & some cubes did not agree with his temperment or stomach.
The older TB was very scatty & had some bad habits. When we got him at 12yrs he was grossly underweight 430-440kg & 16.3hh. We found all mixes & most cubes even the 'calm' ones & weight gain feeds made him worse & lost more weight. In the end we settled on grazing as much as possible, stabled he become demented. Allen&Page Fast fibre, chaff & lots hay. If needed part subsituted with a very small amount of A&P calm&condition. Supplements were probiotic, spearmint, chamomile & another herbal calmer, seaweed or biotin. When he was sold on after nearly 2 yrs he was 490kg that was stable even in winter, summer he would gain a little due to grazing. He calmed down a lot but not all the bad habits.
Good luck in finding the right combination.