West Jet has heated cargo planes, always make sure you get one of those.
Air Canada, and Jazz and the other lines, use that new special form on the off months - September to April - and you can't insure against temperature during those times.
A non heated cargo hold can get down to -30 c when they are way up in the air, even when it is warm down here - and they would not take puppies and cats under those circumstances.
Some of the west jet flights and air canada flights that don't say they have heated cargo (online), actually do have their cargo holds heated with the same air as the passenger part. The planes that fly from Calgary to Medicine hat do not show up as heated cargo on their little list, but the cargo hold is heated, so they can accept cats, dogs, fish and reptiles.