No paperwork is required if the dog is less than three months old if the animal appears to be in good health. A border control agent will make a determination as to the animal's health.
All dogs older than three months must have a valid rabies certificate to cross into Canada. The certificate must be dated and signed by a certified vetenerian, and "it must identify the animal by breed, age, sex, coloring, and any distinguishing marks," according to U.S. Consular Services Canada. Dogs entering the United States must have been vaccinated against rabies in the previous 30 days.
Guide dogs and any other service animals do not require paperworks. Service dogs must be accompanied by their owner when crossing the border to be exempt from the rabies vaccination certificate requirement.
If you are bringing more than a few dogs into Canada you may be asked for if they are your personal pets. The border agent has the authority to determine the validity of a person's claim that they are personally-owned dogs. Canada has specific regulations -- overseen by the Canadian Food Inspection Agency -- for the importation of pets for purposes of resale.
Pit Bulls are not allowed in Ontario. The Dog Owners' Liability Act, instituted under the McGuinty administration, "prohibit(s) individuals from owning, breeding, transferring, importing or abandoning pit bulls" in Ontario.
