Tenant Violated No Pet Clause Then Claimed ESA (emo dog)

[Greg Agius]

I am a California resident. I rented my former townhome through a prop managment company. In late August we notified our tenants who are 1 year through a 2 year lease that we would be moving back to their area from Seattle and would thus be taking care of the property directly. 

Last week we discovered that they have had a large dog on the property since the beginning of the lease. Its an 8 year old dog that I assume they have owned from the start. We asked them to remove the dog and pay 1 year back pet rent. They produced an ESA emotional support dog letter from one of these common websites, dating from just 3 weeks earlier. That's just 2 weeks after we told them of our move. During the 2 previous on site inspections, including 1 from just 2 weeks ago) the dog was carefully hidden and its food dish, etc taken away. 

Do I have any options for challenging the validity of the ESA?