The Homunculus appears to be some kind of small monster armed with a club. When primed, the player will inject a syringe into it which will cause the Homunculus to come to life, and attempt to get out of the players hand. In each new game of Zombies you can only have two Homunculus per team. Once thrown, the Homonculus will yell a war cry, and jump to any nearby zombie and bash their head with the club, killing them instantly. The Homunculus will repeat this by jumping from zombie to zombie for roughly about 20 seconds before disappearing into a cloud of smoke. Though it can attract larger zombies, such as Destroyers, Marauders and the Blightfather, it seems to only be able to distract them but unable to damage them.
Multiplayer and Blackout
The Homunculus only appears when a player performs the "Homunc-a-shuffle" taunt, where it will dance with the player before disappearing in a cloud of purple smoke.
Priming the Homunculus.
On IX, throwing an Homunculus outside of the map towards the crowd will make it cheer, creating "the wave".
On Voyage of Despair, an Homunculus can be seen dancing outside of a map, signaling a code using flags.