Agree with the above feed suggestions. Ultium is good, as are the Triple Crown feeds and Buckeye.
How high quality is your hay? If you feed really good, high protein hay, then you will get a lot better results. Supplementing with alfalfa can help with adding weight and muscle.
Of course, if you are giving a pound or two of Ultium you will not see the kind of results you would have if you were feeding 7 or 8 lbs.
I was sold on the idea of the Pennwoods supplement as suggested by
inguy, but couldn't get my horse to eat it. She won't eat certain flavors they commonly put into horse feed. Cool calories has a sort of fake cherry smell to it, and the Pennwoods has a flavoring too. Most horses apparently love it. I gave the big bucket I bought to a TB at my barn. Haven't heard yet how it is working for her.
I saw a big difference in my horse's hooves after feeding Horseshoer's Secret for about four months. It has a good amount of biotin, copper, zinc, and other necessary nutrients. My horse's hoof walls became extremely thick and hard.