The only way to obtain (Without Commands Or Stuff) this creature (Level 150 Fenrir) is by defeating the Fenrirsulfr (Fjordur Boss).

Right after you the Fenrirsulfr dies, you will get your own fenrir in a cryopod in your inventory.

Dododex might say they are a knock-out tame, you can do that but the problem is they are not able to spawn in the wild, so i guess you use commands to spawn them in and tame them.

More Fenrir Taming & KO Tips