@Elliot4
I agree. I think you should have been able to collect the pet fee and Airbnb should have paid that if guest wouldn't pay it.
Options for the future:
1. Host modifies the reservation to include the pets (guest can still can decline to accept the change or refuse to respond). This can only be done before or during the reservation; not after check-out.
2. Guest modifies the reservation to include the pets and you accept
3. Requesting the normal pet fee via the resolution center
If the guest doesn't pay within 24 hours, then Airbnb should have paid it IMHO. As long as you have proof the guest didn't list the pets on the reservation and photo/video proof they still brought pets. The issue here is the Resolution Center is to cover damages/costs and/or fines associated with breaking a House Rule. I guess in this circumstance, Airbnb didn't feel there were damages/costs/fees associated with the situation?
You can try again to recover the pet fee via the resolution center, but not sure that will have success. Best to simply state your policy/fine for undisclosed pets in House Rules. We also send our Pet Policy to every guest. We either send it after they book and have indicated they are bringing pets, or we send it as part of our check-in instructions as part of this paragraph. You can keep it friendly, but clear:
Pet Policy
We welcome your well-behaved furry friends! Please note: all pets must be listed on the reservation. No undisclosed pets may be on property during your stay. (add whatever other requirements you have like they must clean up after the pet, please don't let pets lie on beds, or left alone at the listing, etc.)
Then be sure you state this in your Addl House Rules along with whatever extra fee/fine you will impose and whether the guests reservation can be cancelled if they choose to ignore your rule.
Bad Review
Be prepared for a negative review as a result of your requiring them to pay for undisclosed pets. This is why you should to add this to your House Rules. If you get a retaliatory review as a result, you should be able to get it removed, as the guest broke a House Rule and then retaliated with a bad review. It's a pain to do this, so do everything you can to prevent it using Add House Rules, Guest Messages and making clear your Pet Policy and fines if guest doesn't comply.