Here's the answer from the Ontario Landlord and Tenant Board website.

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Yes, if a landlord has a "no pets" policy and they learn that a person applying to rent a property has a pet, the landlord may refuse to rent to that person.

The Residential Tenancies Act states that any clause in a lease that prohibits pets is void. This means that once a person becomes a tenant, if they have a pet even though the lease says pets are not allowed, the landlord cannot evict the tenant just for having the pet. However, the Landlord can apply to the Board to evict a tenant if the pet is causing a problem.

For more information on this topic and other rental questions view the Landlord Tenant Board Website.