OK, so I am confused on this and still am after reading the comments. Duo did take "What eats horses?" as an answer, but said "What do horses eat?" is also right. I thought Google Translate could help, so I typed in "What eats horses?" and got "Hva spiser hester?" But when I translated it back, it says "What do horses eat?"
So, my confusion comes from this. If I wanted to say, "What do birds eat?", I'd say, "Hva spiser fugler?" But how would I say, "What eats birds?" If it's the same, how would I know the difference? Would it just be context clues?