”Direwolves,” Bran said. Still half-grown, they were as large as any wolf he had ever seen, but the differences were easy to spot, if you knew what to look for. Maester Luwin and Farlen the kennelmaster had taught him. A direwolf had a bigger head and longer legs in proportion to its body, and its snout and jaw were markedly leaner and more pronounced. There was something gaunt and terrible about them as they stood there amidst the gently falling snow.
— Bran Stark, A Game of Thrones

Legends have it the blood of the skinchangers, or wargs, runs in the veins of the First Men. The children of Lord Eddard Stark find a litter of five direwolf pups and one, the pups’ mother dead with an antler in her throat. The Stark children adopt these fierce little beasts, as it is clearly an omen, for the Stark family sigil is the direwolf.

The children grow to form special bonds with each of the direwolf pups, and these animals become crucial to the ongoing story of the Stark household. As the children grow and mature, so do their direwolves, shaped alike by experience and tragedy. When they are first discovered, the pups are barely a handful, dependent entirely on the Starks for survival. As the direwolves develop, however, they become powerful companions and extensions of the Starks’ spirits. The direwolves and children are extremely sensitive to one another, and much can be read in their behaviour.