My Old Landlord Didn't Disclose Missing Payments, And Is Now Making My Life Hell

For context, this isn't about rent payments. This is about PET rent payments. When my roommate and I first moved in, rent was $850. I was informed by the landlord that I only had to pay for one pets worth of pet rent, which, honestly, I should have gotten in writing.
We were good tenants, good neighbors, and we always made payments on time. When we moved out, we expected to get our deposit back, and instead, we got a bill for someone else's missing payments. Then, once we got our own bill for missing payments, it turned out we had been lied to and our pet rent was supposed to be a separate check. It was never disclosed to us, nor was the "missing pet rent" from our first day until our move out. Now they're tacking on late fees to every single payment, despite never even informing us these existed in the first place. I feel like there has to be some kind of law about disclosing payments like that, or giving your tenants notice about late payments. I know when it comes to rent, there is a requirement. But does that also go for pet rent?
I've been trying to get answers from my former landlord as to why she didn't tell me about any of this just to have it in writing, but she's very evasive towards any answers. Is there anything I can do?
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