If the lease allowed the pet it still must be kept properly. Normally a pet deposit is required up front; however if you are not keeping the pet properly the landlord may be able to declare a default and as part of a "cure" require a pet deposit or other rules. You should have an attorney review your lease for both the permission and any pet rules. If you are breaking the rules the landlord may be able to evict unless there is documented medical reasoning for having the pet. An attorney can help you figure out whether this is possible.