What happens to the animals ... this one is correct This is a subject rather than object question.

You can use both with slight differences. If you use "What happens to the animals" it's perhaps a general question. This structure is used more commonly than the second one. When we add 'do' + main verb, it's called emphatic form. It's used for extra emphasis. Generally in spoken English we'll pronounce the verb 'do' with extra stress as well. So if someone asks 'What does happen to the animals' they are perhaps asking with emphasis or deliberation. I imagine this sort of question could appear in a conversation about animal rights or something like that. For example: A: Cosmetics are tested on animals. B: What does happen to the animals? - perhaps here the speaker is trying to convey some concern for the animals well-being. So although both forms are correct, the first form is used more commonly.

Hi, Carlo! We say "What happens to the animals?" This is because the question is about the subject, and in these cases no auxiliary verb is used. [Subject] + verb [X] happens to the animals. [This] happens to the animals. What happens to the animals? More details: http://www.bbc.co.uk/worldservice/learningenglish/grammar/ask_about_english/080610/ I hope that helps!

What happens to the animals. - if it is an ongoing thing. What happened to the animals. - if it is in the past. :)