I too, am a professional pilot--45 years as a pilot, 40 years as a professional, 25 years with an airline, current Captain. Canyonblue is right. There is not, nor has there ever been an airplane with a pressurized passenger cabin and an unpressurized cargo hold. The entire fuselage structure is a pressure vessel, and has to be that way (try to imagine a balloon with an unpressurized half!). Now some airplanes may have a small compartment in the very front of very back that is outside the pressure vessel, but I don't think PinkyMacGodess is arguing that.
Passengers, their bags, and pets do indeed always fly in pressurized, temperature controlled compartments. What some cargo compartments lack, is air circulation. By keeping fresh air out, older airplanes complied with fire safety standards of the day–were a fire to occur it would quickly consume the available oxygen and suffocate itself. Modern airplanes circulate air in the cargo compartments, and are equipped with smoke detection and extinguishing systems. It's very important for airlines to not put pets in the older style compartments, and we have multiple checks and balances to prevent that. But I can't argue that it hasn't happened in the past. I don't know of any airliners of that kind that are still in service.