If you have any horse rescue centres near you don't be afraid to ask them as they will know a lot of great supplements :) I fed my rescue mare hay, lucerne chaff, oaten chaff and vitamins and within two months she looked AMAZING! Otherwise here in Australia we have a great brand called Teese Trades Stock feed (C3 feed) and that's what most rescue centres use here. Good luck!
Don't over do it on the grain because they shouldn't get too much nutrients but try heavy oils like coconut or sunflower. You can also work them consistently to pick up muscle along with a muscle supplement.
I've done a couple advanced equine nutrition course you need to feed small meals frequently have a proper feed program in place ! Allow as much forage as possible! If bringing back from extreme state please contact a vet ! Also most feed companies have nutrionist you can call and talk to that can recommend a proper grain for energy level age weight ect hope this helps