Garm Wolfbrother

Garm Wolfbrother was the last chieftain of the Thunderlord clan in the Blade's Edge Mountains. His spirit lives on at the Thunderlord Stronghold and in a questline helps guide you to Rexxar's lost family and brethren in the Mountains; the Mok'Nathal.

He starts the quest.

See Blade's Edge Mountains NPCs.

Inspiration
Garmr (or Garm) is the hound that guards the way to Hel [sic] in Norse mythology. During the Ragnarök, Garmr will do battle with the god Tyr, and both will perish at each other's hands in that battle. His role is also described as being similar to that of Cerberus, the three-headed canine guardian of the underworld from Greek mythology.

Related to Fenris the Hunter?
In the novel Beyond the Dark Portal, Fenris the Hunter's given name is "Wolfbrother". According to Rexxar (who ends "The Spirits Have Voices", the quest that Garm gives), Garm Wolfbrother was the last of the Thunderlord chieftains, and is thus presumed as Fenris' successor. It's possible that Fenris and Garm were father and son, or brothers.