Oats and beet pulp are both great options for horses with soy and corn allergies. Since soy is so high in protein, you'll need a protein replacement. Alfalfa pellets or cubes are a good option, and oats have a fair amount of protein, but should probably be crimped or rolled for an older fellow to digest easily. Corn is high in carbohydrates (energy), so you also need an energy replacement. You could use some fat, like an oil without corn or soy in it, oats, beet pulp, or molasses. Another idea is to talk to your local feed dealer and see what they have that doesn't have corn or soy products in them. They might not have much of a selection. You can also contact Purina, or another name brand horse feed manufacturer and ask to speak to a horse nutrition consultant. Good luck!