Dog is Katherine Valentine's pet wolf. Thaddeus Valentine found him when he was just a pup and brought him back for his daughter. Katherine, at her young age, could not tell the difference between wolves and dogs, and called him a dog. The name stuck and became his real name. Katherine takes him around on a leash wherever she goes. She can take him anywhere because all the buildings in London only have no dog signs. Dog is usually happy, no matter where he is. During the climax of Mortal Engines, Dog is frightened by the Guild of Engineers. He growls at them, and although Katherine tries to tell the engineers that he is harmless, they shoot Dog and kill him. Afterwards Katherine would often look down to where Dog usually was, only to find he wasn't there.