You can add something to your description that specifies your charges per pet, and then you can add that by either:
a) making a special offer that includes the nightly rate, any extra guest fees, the pet charge and the cleaning fee (all if applicable)
OR
b) let the guest book as normal, then submit a "change" to the booking where you can add an extra fee after the booking is already confirmed. You will make the change (whether to the guest count, check in/out date, total value of booking, etc) and then it's submitted to the guest for approval, then you will get a NEW booking confirmation.
Those are two options for including a variable pet fee.
Good luck.