A number of entries in the army list allow characters to be accompanied by Fenrisian Wolves. The Wolves can be represented by any suitable wolf model from the Citadel range of models. They must remain within 2" of the character at all times and move at the same rate as him (in effect the Wolves and the character form a small unit). Because the Wolves and the character form a unit they have to take Morale checks for suffering 25% casualties in a turn and will be worth victory points if reduced to half strength, etc.
The character and Wolves may join other units. Treat these mixed units as one large unit led by the character. The Wolves must still stay within 2" of their master, but otherwise count as being part of the unit and may be removed as casualties in place of the character or in place of models in the unit which they have joined.
Fenrisian Wolves may be transported in a vehicle in the same way as a Space Marine in power armour. Characters accompanied by Fenrisian Wolves may not be equipped with jump packs but may ride a bike, as the Wolves are able to run alongside. Fenrisian Wolves always move at the same rate as the character they are with.
Fenrisian Wolves benefit from the And They Shall Know No Fear, Acute Senses, No Matter The Odds and Counter-Attack special rules. Fenrisian Wolves may not use the optional Voluntary Fall Back rule.
If their master is killed then the Wolves will drag away his body to a place of safety and so are removed from play as soon as he dies. In games where the enemy are allowed to 'capture' characters, or can remove things from their dead bodies, they may not do so if any of the Fenrisian Wolves that accompanied the character were able to drag him off (ie, at least one was still alive when the character was slain).
Errata — Chapter Approved: Codex: Space Wolves clarifications (v1.0): Fenris Wolves can ride in a transport along with an Independent Character but cannot 'run along' outside the vehicle.