A good business practice for landlords is to include a pet exhibit with every lease. To some, this may seem unnecessary if the tenant has "no pets"; however, a strong and well written pet exhibit is beneficial for the landlord even if the tenant has "no pets" (note the quotation marks around "no pets").
A well written pet exhibit should state clearly (ideally in large bold type) whether or not pets are permitted. If pets are permitted, the pet exhibit should included a description of the permitted pet(s) and state clearly that no other pet(s) are allowed. If pets are not permitted (or a pet other than the pet(s) described in the pet exhibit is discovered), the pet exhibit should state clearly the consequences (up to, and including, eviction) facing the tenant for the unauthorized pet.
Having a separate pet exhibit that goes with the lease plus having very clear language (ideally in large bold type) in the document indicating whether or not pets are allowed helps prevent miscommunications on this very important landlord/tenant issue. It also minimizes the "I thought a pet was OK" or "You never told me I couldn't have pets" tenant argument, when a lease is silent on the issue or the clause prohibiting pets was buried in a ten page lease.
While most landlords and tenants are good, honest and reasonable people, there are a few that are not. When a tenant with "no pets" moves in and then three weeks later the landlord discovers six pit bulls in the backyard, the landlord will be very glad to have a clear, unambiguous written document to aid in getting the six pit bulls removed.