As you're done taking in the creature's appearance, you notice that Eric is looking at you. Trying to read from his face, you believe he's come to the same conclusion as you.
Fenrir.
You don't have to be a mythology junkie to recognize that. The two of you face Campbell with newfound sobriety.
[/spoiler]
Plot twist: It turns out to be either Hati or Sköll, the other god-monster wolves of Norse mythology.
Unfortunately we are not mythology junkies, so we don't know that
