Trailer is the resident dog of the Warehouse, and often acted not only as a canine agent but an unofficial mascot for Warehouse 13.
Trailer is an English Shepherd discovered as a unnamed stray by agents Claudia Donovan and Steve Jinks in Wisconsin after an artifact snag. He was brought to the Warehouse on Artie's instructions as a way to determine who had stolen the artifact from Claudia and Steve and tranquilized them during the fact.
Artie used Mr. Mental and Coco's Fezzes in an attempt to retrieve a license plate number from Trailer's mind, but before Artie could finish the numbers Trailer was distracted by the renegade Scarab of Imhotep and chased after it. This left Artie unconscious and Claudia and Steve with the responsibility of finding Trailer in the expanse of the Warehouse. Claudia eventually came up with the idea to use Pavlov's Bell to summon Trailer, and he appeared in turn, having caught the scarab and still wearing the fez.
After the plate numbers were retrieved and the fezzes removed, Artie explained that the physic connection was just as strong even without the artifact and so Trailer would have to stay at the Warehouse. Artie also revealed that his name was "Trai", short for Trailer, and that he owned the Warehouse now, as dogs own humans whereas cats are prisoners.