What a Canadian carrier can ask you for
Under ATPDR, a carrier is allowed to make one reasonable request: documentation that your service dog was individually trained by a qualified trainer.

Photo: Egor Gordeev / Unsplash
The single request
This is not a demand for medical records or a diagnosis. It is confirmation that the dog was individually trained to do tasks that assist with your disability, provided by an organization or person who specializes in service dog training.
How to be ready
Carry the documentation in a form you can show quickly at check-in or the gate. Two supports produce it together: a clinical basis for the disability related need, and a qualified trainer attestation of the dog's individual training.
Advance notice
Some carriers ask for advance notice. Provide it when you book so the carrier can assign appropriate space. Notice does not change your right to travel.