After quite a bit of research and some opinions, I decided to go with alfalfa pellets instead of any type of grain. Alfalfa is a type of hay that is very high in protein. The pellets are basically a compressed version of the alfalfa hay and do not contain any types of grains or sugar of any sort and there is very little processing necessary.The recommended amount to feed a horse that is trying to gain weight is about 1 pound per 100 pounds of horse. This would mean that Santana would need about 11 pounds per day. Once she is up to a good weight (she is almost there now), you can back the amount down to about 8-9 pounds per day.
I also added ground flax seed to her diet. Many people eat this as well. Once ground, the flax seed is a light colored powder that is just sprinkled on the top of her pellets. She is getting about a cup of this per day currently. Once she is up to good weight, I will back this down to 1/2 cup per day. The flax seed adds "good" fat to her diet to help her gain/maintain weight. This also helps tremendously with her coat! She will be gleaming once she sheds her winter coat!
I never knew a horse's diet could be so complicated! Back in the day, my horses were super easy keepers. I never really had to think a whole lot about what I was feeding them because they always stayed fat and happy.