Personally, I can't imagine someone making a reservation and not telling you they are brining a Service Animal and/or an Emotional Support Animal (SA/ESA), however both are legally allowed in your rentals. There are requirements that need to met. You may ask 2 questions:
(1) is the animal a service animal required because of a disability?
(2) what work or task has the dog been trained to perform?
Neither SA or ESA are to be left alone in the unit at all. Meaning when the guest/handler leaves the animal goes with them. The SA or ESA have to house trained. These are not pets. They are service animals. They are to be either leashed or under control at all times. The guest does not have to reveal what their disability is. And there is NO documentation to officially designate an animals as a service one. People can fake any document they want...
The reason I use the word "animal" over "dog" is because according to the ADA a SA or ESA can ONLY be a dog or a small horse. So there is no such thing, according to the ADA, as a Service Snake or an Emotional Support Cat. And if I live in the home that I Airbnb and have severe allergies and/or asthma the ADA does not care. My stance is that too many people are taking advantage of the ADA rule, calling their pet a service animal. Maybe it's time for the ADA to rethink some of their wording instead of using the honor system. How lower can you go than impersonating a person with disabilities to get your pet in a rental unit. My building does not allow pets other than unit owner pets, but SA and ESA are allowed for renters.