Answer: I'm assuming from the tone of the question and the comments that you don't like the rats and that your dislike of the rats is the core reason why you don't want them in your apartment.
For my part I don't blame you, rodents are gross. From your question it sound like the pet clause in the contract is a just something your using to justify feelings that you already have. It's usually more helpful to focus on the core of the issue. That you don't like xyz about having the rats around.
The big problem your going to run into when talking to your roomate, is that you and your other room mates already agreed that she could keep the rats if she kept them in her room. Changing your mind now and asking her to get rid of her rats is going to be viewed by her as you breaking your word.
The best way to handle this is to be open (and polite) about how you feel. And own your own behavior. You can own your own behavior by telling her that you made a mistake. So you could say something like this:
I'm so sorry I know we already had an agreement about your rats, I just didn't realize that I would be unable to handle xyz.
xyz should be something specific that you find hard to handle about the rats. ie. smell, seeing the rat poop in your common areas(bathroom, etc. . ), not being able to sleep at knight knowing there are 8 rats next door. What ever it may be, it should be a specific and real concern that you have.
1) By apologizing for not being able to keep your agreement, you are more likely to diffuse any hard feelings (on both sides of the agreement).
2) By phrasing this as a problem you have handling the rats you avoid attacking her and making her get defensive, which usually leads to arguments.
Personal advice: Try to keep your self open for solutions other than "the rats must go." The example text above is designed to promote a conversation about a specific problem. If you remain open to new solutions you may very well find a way that she can keep the rats with minimal discomfort to yourself.
Be forewarned she likely loves these rats. People don't part with things they love easily (if at all). If you push for removal of the rats, there will be hard feelings.