Grass, hay, oats, grain, carrots, apples, sugar cubes anything of that nature!
There is a difference in what a can eat and would like to eat. ;)
A horse's likes and preference will depend on the horse. A horse can eat bananas. I tried feeding them to my horse - nope. She likes plenty of other things though. She enjoys: apples, carrots, grapes, and raisins, but no bananas :(