A Martian possessing Marcus Aurelius Belt.
Martians are aliens native to planet Mars. (TXF: "Space")
Biology
Martian entities resemble the Face on Mars, vaguely human, but are incorporeal. They appear to have the ability to fly and are parasitic. The entities are able to survive without a human host, but appear to prefer them, entering and leaving at will. An afflicted human's face may temporarily and spontaneously appear as that of the entity. Some form of powerful bond exists between entity and host. (TXF: "Space")
History
On an extravehicular space walk in 1977, a Martian entity attached itself to Lieutenant Colonel Marcus Aurelius Belt. Realizing an entity plagued him and where it had come from, he sabotaged what Space Shuttle missions he could, during his time at NASA, in an effort to prevent others from becoming similarly infested. An exhaustive internal struggle with the entity caused Lieutenant Colonel Belt to collapse. The alien attempted to flee but an example of the strong bond between entity and host was exhibited by Belt's physical struggle, in a hospital room, with the entity which occupied him; the entity made an effort to the contrary but Belt was able to keep it attached to himself temporarily. When Belt threw himself out a window in an attempt to prevent the entity from infesting another, the entity died along with Belt himself. (TXF: "Space")
Trivia
The Martians of The X-Files have some qualities similar to the Gelth of Doctor Who and a multitude of incorporeal entities in Star Trek.