A horses main diet should be hay unless pasture is available. My kids are on hay all winter and grass come May or June. I start them on the grass diet gradually over a week or 2. If you let them out on grass all at once they get sick. Same with any feed change with horses, they get upset tummys easy :)
I feed a pelleted feed, but that is determined by the individual horse, work load, health etc. They sell mixed feeds, sweet (usually molassas) pellet, all different types. I like simple and my horses are all easy keepers so don't need a lot of extra. Check out purinas web site it has some good info on it. http://horse.purinamills.com/
My kids don't like people food so no apples or carrots for them, they just spit them out, although Vida does like hard candy. I carry it in my saddle bag for myself and she always wants a piece.