The Growler was a dog that was owned by the family of Varamyr Sixskins.
Varamyr, known as Lump at the time, used one of his family's dogs--Loptail, Sniff, and the Growler---to kill his younger brother Bump. When Varamyr's father came upon Bump's body, the dogs were sniffing around it. Not knowing which one had done the act, he put all three to death. However, before Loptail's death, Varamyr slipped inside his skin and felt Loptail's death. His reaction, a scream, informed Varamyr's parents that he was a warg.