Answer:
Horse apples = manure
Yes, sometimes horses eat manure. This is called coprophagia. Foals start to eat manure at about two weeks of age. It is speculated that this is how they get good gut bacteria that will help them digest solid food. Up until then, their diet is totally milk, and so they only have milk-digesting bacteria in their gut.
Wild horses commonly drop manure into large piles or stacks, and often come back to eat it in the winter when feed is scarce.