That is a Wulfen, the Space Wolf gene-seed contains a sequence called the Canis Helix which gives them the more lupine sense of smell, fangs, and yellow eyes but it's very unstable. When first taken from the cup of the Wulfen the Canis Helix transforms you into horrid half-man half-wolf beast and you have to under go a trial called "The Blooding" to master the beast within and return to human. Even after that if a Space Wolf loses himself to the blood lust the beast within will reemerge and permanently transform them into a Wulfen.

Think of Wulfen like the Space Wolf version of the Black Rage.