Cloud Guardian are mobs that protect the player until death. By using a Cloud Staff, a player can spawn two Cloud Guardians. They follow the player, floating over either of the player's shoulders and attacking any other mob that damages the player. Once spawned, Cloud Guardians will continue following the player until killed or otherwise removed. If you use the Hammer of Notch while the Cloud Guardians are next to you they will die, since the Hammer of Notch has such a big hitbox. They drop icestone on death.
They are hidden in the code for now, however it is unknown if they will become fully functional.