Cerberus is the Hellhound that spawns on the walls in various places in MOTD. Cerberus is required to be fed in order to collect Hells Retriever.
The first head is by the B23R if you go to CD Street from the Spawn Room.
The next is in the UZI room in the Infirmary to the left of the door into the Dead Shot Daquri area.
The last one is by the crafting table at the docks below the drop from the Juggernog Machine.
It should be known that the dogs will only eat zombies killed when it can eat, if he is chewing it wont eat zombies killed while he chews, this makes it hard to stay efficient. The head will appear after a zombie is killed in front of it, before this it is a drawing on the wall.