That kind of fantasy probably pretty common, and doesn't mean you're 'not normal' or need professional help. The important thing is that you said it can be "someone else" it doesn't have to be you doing the rape. So I'm guessing it's the violence that gets you off.

Are these fantasies becoming obsessions, are they interfering with your life negatively?

Would you consider your social relations healthy?

If it's not a problem, it's not a problem. That's why we have loli ;). Hell the Japanese invented it, and they're not exactly world-leaders in child abuse.

Useful questions you might want to consider:

What can you say about your relationship with others? How are your friendships/sibling relationships?

Do you have brother(s)/sister(s)?

Girlfriend?
Boyfriend?

Female friends?
Male friends?

What do you do for fun, besides fantasizing rape?

How is your relationship with your parents?