Cerberus

 is a three-headed dog seen in the Zombies map Mob of the Dead and Cell Block in Call of Duty: Black Ops II.

There are three dog-head markings on the wall across the map: in the Infirmary, in the Docks and on the Broadway, under Electric Cherry.

A head resembling a Hellhound will spawn on the wall when a zombie is killed near one. Cerberus will eat zombies killed near it (although if it is already eating one, it will not take a zombie killed during the eating process), and after it has eaten six zombies, it will vanish into the wall. After feeding each of the heads six zombies, a wall in the Citadel Tunnels will open, revealing the Hell's Retriever. One of the three heads is also featured in Cell Block, in the starting room (where the B23R is bought). Since the Hell's Retriever is not available in Cell Block, feeding Cerberus' head enough zombies will make a random power-up spawn and rise out of the lava pit below.

Trivia

 * The player will take very slight damage from going near one of the heads. The damage is so minimal it should not be considered a problem, and a damage directional marker doesn't even noticeably appear.
 * Only non-crawling zombies can be fed to Cerberus.
 * Cerberus pauses for moment after devouring a zombie.
 * Cerberus will only eat a zombie that was killed while it is not eating.
 * Brutus cannot be fed to Cerberus.
 * There are three heads and you must feed each one six zombies, is a reference to the number "666", also known as the mark of the Beast.
 * Above the entrance to the Hell's Retriever room is a piece of sheet metal with three dog-head markings. These markings can also be seen on the walls where a head will spawn from. (as mentioned above) For every head fed, one of the three head markings will light up. Once all three are lit, the wall will disappear, allowing access to Hell's Retriever.
 * Zomies hit with the zombie shield cannot be fed to cerberus.
 * Cerberus is the name of a three headed dog that protects the underworld in Greek Mythology.
 * If zombie's head is shot off and the zombie is eaten by Cerberus, it will have its head back while it's being eaten.