@Meg1428 @Mark116, I'll paste the Airbnb policy below, but it is worth noting that unfortunately, the guest is "not required to disclose the presence of a service animal before booking."
If a guest shows up with a dog, I approach them, usually by phone. It is good to start by pointing out the presence of the dog and asking if it is a pet or a support or service dog. Knowing this is important, because I believe the policy states you can deny emotional support animals, but not service dogs.
We had to ask a guest to leave once because he brought his dog. We decided to give him a full refund because we didn't expect a booking that time of year anyway. My wife has intense allergies to animals and unfortunately for the hour he was there, the retriever left tons of hair behind. Since then, we made the no pet policy more obvious by adding it to the "other things to note" page. Airbnb puts the House Rules very low on the listing's page, so don't just rely on that.
https://www.airbnb.com/help/article/1869/accessibility-policy
Best Wishes,
Jared