Cerberus

The  is a three-headed dog seen in Mob of the Dead in Call of Duty: Black Ops II.

There are three dog-head markings on the wall across the map: in the Infirmary, the Docks and the Broadway (under Electric Cherry). A dog's head resembling a Hellhound will spawn on the wall when a zombie is killed near one. The Cerberus will eat zombies killed near it (although if it is already eating one, it will not take zombies killed during the eating process), and after it has eaten six zombies, it will vanish into the wall. After feeding each of the heads, a wall in the Citadel Tunnels will open, revealing the Hell's Retriever.

Trivia

 * The player will take very slight damage from touching one of the heads.
 * Only non-crawling zombies can be fed to Cerberus.
 * Cerberus pauses for moment after devouring a zombie.