Well, L'espace180, they DO travel in the hold, and the hold is pressurized. There are strict rules regarding how much space there must be around the cage. This is dependant upon the size (weight) of the animal in question. Example: A dog weighing 30kg (like mine does) would require a minimum of 0,6 cubic metres of air around him for a 1 hour flight, 1,1 cubic m for a 2 hour flight and so on. Usually you round upwards to the nearest half/full cubic meter due to possible delays and so on. The "flight time" is for this purpose defined as how long the belly is actually closed, not the flying time. For a 12 hour flight, he would require 6,8 cubic metres! No wonder many airlines have commercial embargos on pets in hold (so called AVIH) on long haul flights. Most airlines recommend that you do NOT sedate your animal. You never know what will happen to the animal, and a sedated animal can swallow his tongue for example. Not all cargo holds have adjustable temperatures. We have a list of our acfts and where we should place animals depending on the a/c type and temperature.
If you have seen BBC´s "Airport", you have seen some good, and some awful, examples of animals being transported.