Landlord lied.

Did you sign the lease? What does it say about pets and deposits? If you signed anything, that would be the basis of how you could go about your problem on the deposit. But if that wasn’t fully explained in the lease, you should know that your landlord has no right to keep the security deposit.

However, on the issue of your landlord lying to you, that can be difficult to prove. A verbal misinformation (whether it be deliberate or not) can be difficult to prove. If you really want to contest it, it’s possible. You can sue your landlord in a small claims court or you can let a mediator come in between. The court option is rather costly but letting other professionals handle it would only cost $39 (if with RPA). It’s your choice.