Answer:
Rats and mice are afraid of any predator. Nearly anything will eat them. Some predators that frighten them include cats, dogs, owls, foxes, and snakes. Rats are also usually afraid of humans.
edit:
Rats are also afraid of the letter "P"
If you say any thing with a "P" in it (Pizza/pop/parlor) they flinch/squeek/run. They are also afraid of balloons. I own three rats, and I'm being serious.