Other than the peak time restrictions, there are also time of year/temperature restrictions on certain aircraft.
December 15 to January 12
Due to increased passenger and cargo loads during the peak winter travel period, no pets are accepted as checked baggage or cargo.
November 1 to March 31 and at any other time when temperature is
0 °C (32 °F)
or below
Cats and dogs weighing
less than 4.5 kg (10 lb) are not accepted in the baggage compartment of any of our aircraft.
Pets are not accepted on the following flights and aircraft:
It is not recommended to transport animals as checked baggage on our Airbus
A319-100,
A320-200 and
A321-200 aircraft: the cargo compartment is not heated and temperatures can be as low as 2 °C (36 °F).
https://www.aircanada.com/ca/en/aco/...ance/pets.html