Do Ihave to follow a rule that the landord says is a rule, even though the rule is not in the existing contract?

You have described a situation where the lease that you and your landlord signed stated that you could have a pet weighing up to 90 pounds.  Evidently, the landlord later said that he told you that the 90 pound limit in the lease is invalid, and that he requires you to comply with a rule that says no pets over 30 pounds are allowed.  In this situation, your rights with respect to maintaining a pet are governed by the terms of the written and signed lease.  If the provision about the 90 pound maximum size of a pet is in the lease, the landlord does not have a right to change it in a way that reduces your rights (such as by saying that you cannot have a pet that weighs more than 30 pounds).  However, the rights of the tenant in these circumstances cannot be determined without a thorough review of the lease and all related documents.  There can be circumstances that may permit a landlord to make such a change in rules.  For example, in some cases, rules like pet restrictions are not included in the lease itself, but are included in a set of written rules and regulations.  In that case, the lease usually contain a provision stating that the tenant is required to comply with the rules and regulations as set forth in a separate document.  When the rules are in a separate document, sometimes the lease will also state that the landlord has the right to add new rules and change existing rules.  If a lease reserves these rights for the landlord, he may have the right to change the rules pertaining to the maximum weight of a pet.  In those circumstances the tenant would be required to comply with the revised rule.