ABulk Cargo Hold
Pets travel in the bulk cargo hold, a special cargo compartment on the aircraft. The cargo hold is usually climate-controlled to maintain temperature, humidity, and pressure similar to the passenger cabin. However, temperatures can sometimes rise due to external heat or sunlight reflection.
Until just before departure, pets are kept in air-conditioned or shaded areas. However, moving to and from the aircraft takes place outdoors, so temperature and humidity can fluctuate significantly depending on the season.
As a side note, when racehorses are transported, veterinarians sometimes accompany them inside the cargo hold.