From A Wiki of Ice and Fire
|Book||A Dance with Dragons (mentioned)|
Loptail was a dog that was owned by the family of Varamyr Sixskins.
Varamyr, known as Lump at the time, used one of Loptail, Sniff or 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 died, Varamyr slipped inside his skin and felt Loptail's death. His reaction, a scream, informed Varamyr's parents that he was a warg.