I am NOT an equine nutritiontist! Please take what I say with a big fat grain of salt.
At the very least, they need salt, zinc, copper, vitamin A and E and maybe cobalt and manganese if those aren't already present in your soil or water. They can also benefit from selenium if you have none in your soil or hay (this is something your dept of Agriculture can tell you about). Also, some form of oil is good for them - a lot of people provide flax seed (ground, not whole because the whole ones tend not to get chewed and are passed whole) for this purpose. Other oils do not have the good omega fats and oxidize too quickly to be beneficial. Some people also feed biotin for the hooves, but I'm not convinced it's necesssary. Hemp oil and camelina oil are good things to add as well.
I believe in keeping things simple so I have a custom-mix mineral supplement which I mix with beet pulp (no molasses) and timothy hay cubes (you can get timothy-alfalfa, but most horses don't need alfalfa which has higher protein), and feed hay and plain white salt (even my salt licks are plain white). Because what you DON'T feed is, in my humble opinion, just as important as what you DO feed. I don't want to feed any extras or fillers my horses don't need and which could even be harmful. I don't want to feed iron because we have too much iron in our soil, water and hay, and too much iron is very bad for horses.
But I am not an equine nutritionist. I am just trying to learn as much as I can and I don't want to buy pre-mixed feeds or ration balancers because they are a one-size-fits-all option, but not all horses are the same.
So I start with a hay analysis and build from there, working with an equine nutritionist. Just knowing what they minerals they need isn't enough because there are ratios you need to follow for proper absorption of minerals. Equine nutritionists don't charge a lot, a hay analysis is very cheap (about 30$) and then you have a plan that is adapted to your horse and your environment.