Pets: A landlord may evict a tenant for keeping an animal if it:
caused undue damage,
caused a serious allergic reaction in another person or the landlord,
interfered with the normal enjoyment of the property by another tenant or the landlord, for example, causing undue noise,
acted aggressively to others – thus affecting their safety, or
is a species or breed of animal that is inherently dangerous
http://www.orht.gov.on.ca/userfiles/..._3_7968_1.html
Even if you owned a pit I would assume that the dog wouldn't become
inherently dangerous until the law kicks in. Of course that may be one of the first cases to hit the court system.