What Would You Feed A Horse?

We're planning on getting a horse and my dad wants to make sure we're feeding it the right food. I think we just need to make sure it has fresh water, a salt lick, and hay/grazing pasture. I don't think you're average, 12 year old horse needs special food, does it? Unless it's sick, underweight, or old, you don't need to buy bags of food. Am I right?