The three rats that run out in front of the zombie before that are not hostile either. That also took me a long time to figure out.
One of them (the one that fights the zombie, if it survives) generally turns hostile for me. The other two remain non-hostile.
You can "herd" one of those rats ahead of you along the passage, if you sneak. As long as you're behind it and sneaking, you will train Sneak. My characters often advance Sneak to Apprentice by the time they get to the "goblin zone," simply by following that rat all the way to the end door, sneaking all the way.