@Harry477 You inform Airbnb for them to take care of the situation, either they take the pet out of the house or pay for whatever fine that you impose for the emergency situation. I don't think guests take their time to read the house rules or maybe glanced through what's ok for them.
@Harry477 As long as it is a pet and not a service animal you say that they cannot stay and ask Airbnb to cancel their stay. Sadly you will lose the income from said stay but at least the pet free house will stay pet free.