Fenrus was the pet of Arugal, Archmage of Shadowfang Keep. Fenrus prowled the main study of Arugal's chambers. He was unbelievably large, and it is possible the worg was subject to dark magic at the hands of Arugal or the worgen. Whenever Fenrus detected a player, he let out a thundering roar that could be heard from everywhere in the instance. Arugal would come to see adventurers off when they battled this Worg, summoning 4 of Arugal's Voidwalkers which you would have to kill; these were non-elite and didn't hit hard.
Fenrus cannot be tamed.