Fenrirs come in two variants, normal white Fenrir and larger and stronger Black Fenrir. Normal Fenrirs have very little health. It only takes one grenade or a couple pistol rounds to kill them. Black Fenrir can take almost as much damage as a basic Ooze. Fenrirs are dealt extra damage when shot in the gaping holes on their sides. Fenrirs will follow B.O.W. Decoy grenades when thrown usually getting hit by the explosion when it detonates.
Fenrirs' are able to run fast and will circle around the player when close. Their only attack to leap and bite the player. If hit at a far distance, the player will only receive a small amount of damage. If they hit the player at a close distance, then will knock the player over and continue mauling them. During this the player must shake them off via quick time event. In Campaign, Black Fenrirs can howl, which echoes and summons more Fenrirs.
The associated mission for defeated Fenrir in the 3DS version is called "Defeat Fenrir (Defeat 20)" which has the description "Biological weapons based on wolves. Defeat 20 of them." which yields 10000BP. Both types count towards the defeat total.
Both types of Fenrir count as "Beast" type enemies effected by the "Hunting Gear" custom parts in the port versions.