Thetans are normally peaceful spirits that can possess Pygmies, added in Ep 47: Apocalypse Ow! They are created when a Pygmy is put on the machine in the Closet Room and caused to explode Thetans.
Thetans appear to be red blobs with two eyes that randomly float around the screen. Angry Thetans, however, appear to have a permanent frown.
Calm Thetans will float around the screen, always facing outwards, towards the outside of the screen (and presumably towards you). When Tom's cell is opened, the Thetans will open the door and allow you to pick him up.
When calm Thetans possess Pygmies, they become depressed and say "phew". The thetan will then exit the Pygmy. When pygmies are possessed this way, nothing deadly will happen to them.
When Xenu is summoned in the Xenu Altar, angry thetans will appear. These Thetans will signal the Apocalypse and (along with a red tinted sky) the Thetans will keep floating around the screen until the Apocalypse ends. Any Thetans created by the machine will act as Angry Thetans, even if they appear as Calm Thetans. When a Pygmy is possessed by an Angry Thetan, the Pygmy will take the appearance of a vampire (though slightly different) and randomly attack Pygmies, excluding Charlie when he is not possessed.
Possessed vs Normal
When an Angry Thetan-possessed Pygmy attacks a normal one, they will jump on top of them and either viciously punch them in the face, or bite half their face off. In either interaction, the victim stands up, staggers around for a little bit, and falls to their death in the water or off a cliff. The attacker will then pretend to feel guilty and say "Oops, my bad!" and then shake his head to expel the Thetan from his body.
Possessed vs Possessed
When two of these Pygmies come across each other, they will brawl, taking turns randomly punching each other and then having a slap-fight, which leaves them both bruised and ends the animation. After this, the Pygmies will shake their heads and will revert back to their normal possessed selves, and will not die or expel the Thetans from their bodies.
- Thetans are Scientology's equivilant of spirits or souls found in other religions/beliefs.