Skoll is a large blue saber worg Spirit Beast that is a rare spawn, appearing every 8-16 hours in one of three know locations of Storm Peaks.
Skoll is notable for being the first Spirit Beast to not be feline in looks. He is also known for the lightning effect that crackles around his body.
He's an exotic pet and certainly one of the most spectacular pets a hunter can get, looking great in 'regular' and in prowling mode. Like all Spirit Beasts, he requires the 31 point Beast Mastery talent to tame.
The value of killing and looting Skoll is minimal, compared of his live value to a lvl 80 hunter. BM specced hunters value the occasion to tame Skoll as much as any epic item you would give them.
In the rare event you would spot him, make sure you whisper your hunter friends or guildies. They will be happy to reward you.
In Norse mythology, the aspects of the sun and moon are pursued through the sky by two wolves. Skoll is the wolf that pursues the sun and, at the onset of Ragnarok, will finally catch and devour it.
Skoll's brother, Hati, will do the same to the moon.
Note
Despite being a rare creature, Skoll is not on the list of creatures needed for the [Northern Exposure] or [Frostbitten] achievement.