I can relate with your comment about animals getting into the duct work for central air as well.

If the person has a service animal,  they know. 

The leases are clear about animals of all kinds and we have not had any problems.

Two different animals. 

If the dog is not causing any damage, just let it go.In the future, you should require the Tenant to kennel their animals or remove them from the property any time you visit or send in a contractor.

They have an unauthorized animal that is potentially causing damages that may cost you hundreds or thousands of dollars when they leave, and they completed renovations on the home without permission and without following the lease or the law.

We require that we meet the animal, and then charge $150 non-refundable fee, and we raise the rent $50 per month.

This might help him judge the temperament of the dog.Providing fencing (privacy fencing would be useful) in the rental unit might also increase the sense of security.Everyone thinks their pet is the "sweetest" animal around; even those testifying in court after their sweet pet has attacked another dog or human. 

If you're buying a quad it's a completely different animal. 

We are looking for a 3-5 acre plot of land to raise small animals, grow produce and raise a family.