It's not worth the risk, pay for a ticket and ride in the cabin

The answer is it depends on the plane and how they configure it. Some have multiple holds, and not all of them are heated, but they would of course put animals in the hold that was.

If you are asking to try and keep something cold, I would not count on that either. It's going to take a long time for luggage to get cold on a plane if it's in the middle of all the other bags in the hold.