The Zarn is an arrogant, anti-social alien creature shaped like an invisible humanoid with a pattern of bright dots of light on his skin. He is also able to turn completely invisible at times, to read minds and use telekinesis. He appears as an antagonist in a few episodes of the second and third seasons of the original Land of the Lost.
He claims to have been a scientist specialized in the study of humanoid life, before him and his ship were trapped in the primitive pocket universe realm known as the Land of the Lost. He lives in his semi-functional ship in a dark, mist-covered area which the Pakuni are frightened to go, perhaps indicating that he's been there since way before the Marshalls' arrival. He is extremely sensitive to the emotions emanated from human beings, and the strongest feelings can cause him physical harm, especially things like anger, sorrow, and above all pity. The Zarn technology gives him the ability to create robotic servants of many kinds, ranging from a metallic dinosaur-like guard to organic-looking humanoids equipped with artificial intelligence and telepathy.
In his first appearance, the Zarn creates an android shaped like a beautiful woman and programs her with some of Rick Marshall's memories (which he had access to due to telepathy) to make him believe she was a girl who grew up in the same city as him. He tricks Marshall into believing that and observes as he becomes romantically attracted to the android woman as part of a scientific experiment.
In another occasion, the Zarn attempted to use the gravity drive of his ship to escape the Land of the Lost, unaware that that world is an artificial universe built in such way as to be virtually impossible to escape. The use of his ship's gravity drive interfered with the Land of the Lost's own gravity system and threatened to destroy the whole world, including the Zarn himself, but even when he became aware of that he still attempted it. To prevent the destruction of the land, Rick and Will Marshall used anger to harm the alien, causing him to tremble in pain and lose control of his ship's functions. As the gravity increased excessively, the Zarn's ship collapsed, destroying any hope of him ever escaping. As the Marshalls expressed genuine sympathy at that, they inadvertently caused further harm (as they weren't aware at that point that the Zarn was injured by the feeling of pity as well) and the furious alien demanded to be left alone.
2009 Movie
A very different version of the character appears in the 2009 film. In this movie, he's a Sleestak and the ruler of his species. He was assumed to be the main antagonist, since Enik claims that he plans to send his army invade the World. However, one of the protagonists finds a hologram message from The Zarn revealing that he's innocent and that Enik is the one who planned to send the Sleestaks invade the World. The hologram then shows The Zarn being killed by Enik.