Thank you for the information and your question. Oddly enough, under Washington landlord tenant law, assuming the landlord has complied with the law as it relates to taking a nonrefundable pet fee, there is no requirement that damages caused by a pet be charged against the fee. This is because it is actually a fee for the purpose of giving permission to have a pet. Since it is nonrefundable from the beginning, there would be no requirement that any damages be charged against the fee. It can definitely be a windfall for the landlord, but there are cases when it works otherwise.
Therefore, assuming that the landlord has withheld proper damages from your security deposit, despite them being caused by a pet, they would be acting within the law. However, you might have a basis to object to the withholding if the charge is not reasonable or there was no walk through or check list. You can see more details about your rights and options (too numerous to list here) by going to the following links:
Please feel free to ask for clarification after you have had a chance to read the information at the links I provided. If none is needed, then if you could take a moment to leave a positive rating in the ratings box above, I will receive credit for assisting you today. Thank you