Airbnb does not want a pet fee. This was made clear by their policy change last year. If you want to charge a fee for pets (and I do) it must be stated in your rules or it is not enforceable. Then, after the person books or requests I send a modification or a special offer.
Almost all my people bring pets, so I do it almost every single time. It’s not that hard and none of my guests have complained. And before you ask why don’t you just add the fee into your rate, I don’t because that throws me out of range with my competitors. I’m booked as much as I can be so it works for me.