The Fenrir is a type of B.O.W. that appears in Resident Evil: Revelations. Named after the giant wolf Fenrir from Norse mythology, these creatures were created by Il Veltro through infecting wolves with the T-Abyss virus. Fenrirs are very similar to the Cerberus creatures created by Umbrella: both are canines that undergo a severe state of decay upon infection. In the case of most Fenrir specimens, their bodies appear to have been torn open from the inside out as the mutation causes the wolves' ribs to shift and stretch outward, making it appear that the creature has been hollowed out. Even missing most of its vital organs, a Fenrir is still capable of functioning, thus it is classified as 'undead'.
Fenrirs are savage pack hunters and they do not tire, so they will relentlessly pursue prey for as long as it takes to catch them. Packs are led by larger alpha males known as 'Black Fenrir' which are considerably stronger. As with other undead B.O.W.'s, Fenrirs can only be killed permanently by destroying the brain.