We were not allowed any pets other than fish in the dorms. Nonetheless, several people had cats.
One guy in my dorm had a cat whom he named Fish. That way, when asked if he had a pet, he could truthfully say, "Just Fish."
We also had a house manager who would selectively enforce the rules. If you had a cat and he liked you, you got to keep your cat. I wouldn't be surprised if money changed hands.