You have started feeding your horses naturally by feeding them apple cider vinegar. I prefer organic apple cider vinegar for my horses that way you don't have to worry about pesticides/chemicals.
Also it is good to use vinegar made from the whole apple.
Apple cider vinegar has calcium, iron, magnesium, phosphorus, potassium, sulfur, silicon, chlorine and sodium. You can feed an average dose of 1-2 ounces a day.
It is good for cleansing blood impurities, reducing calcification in joints and arteries, tying up of muscles and improving overall health.
It helped my horse Penny with her clicking joints. If you want to stay with natural supplements you can also add organic kelp.
Kelp has calcium, iodine, iron, magnesium, manganese, phosphorus, potassium, selenium, sulfur, zinc, vitamin A, Vitamin B, vitamin B12, vitamin C, vitamin D.
For horses you can feed 5-20 grams a day (1 teaspoon = 5 grams, 2 teaspoons = 10 grams, 3 teaspoons = 15 grams, 4 teaspoons = 20 grams).
I feed my horses 10 grams a day plus 2 ounces of organic apple cider vinegar a day.
You can't go wrong if you include both of these in your horses daily diet. It can't get anymore natural then that.
Best of luck.