UFO Pilot

Episode: Fallen AngelThis alien is the pilot of a downed UFO that crash lands in Wisconsin. The military tries to cover up this event by telling Scully that it's a downed Libian jet with nuclear warheads, but Mulder comes to believe that the government is using technology gleaned from these "fallen angels" to build top secret aircraft.

Bounty Hunter

Episodes: Colony, End Game, Talitha Cumi, HerrenvolkThis alien's main objective is to destroy clones, such as the Gregors, the Samantha clones, and Jeremiah Smith before they can expose "the project." He also appears to have healing powers, which the Cigarette Smoking Man orders him to use on Mrs. Mulder

Arctic Worm

Episode: IceScientists at an Arctic Ice Core project are driven to madness and wind up murdering each other after they are exposed to these worm-like prasites that they unearth. Scully eventually figures out that two of the creatures in one host will kill each other, enabling the agents to get out alive, but more of these worms may still be frozen deep within the ice core.

The Oilien

Episodes: Piper Maru, ApocryphaReleased from the remains of a WWII squadron on the bottom of the ocean, a being of deadly force assumes a human guise. Alex Krycek unknowingly escorts this alien into the U.S. The alien entity leads him to his UFO, and re-enters it's craft, pouring itself from Krycek's eyes and mouth, thus earning itself the nickname "The Oilien."

Foo Fighter

Episode: ApocryphaThis is the alien spacecraft found on the bottom of the Pacific Ocean, downed by American fighter pilots during the second World War. Missions to salvage the aircraft failed due to intense radiation. It carried a being that we would soon come to know as the Oilien.

Alien Listeners

Episode: Little Green MenThese extraterrestrial beings make contact with an abandoned radio telescope in Arecibo, Puerto Rico. Tipped off by Sen. Richard Matheson, Mulder travels there to investigate. He encounters the aliens, but once again is left with no physical proof of their existance.

El Chupacabra

Episode: El Mundo GiraThis creature out of Puerto Rican folklore kills those it comes in contact with by inflicting a deadly fungal infection upon them. Mulder believes the enzyme carried by El Chupacabra could be alien in origin. The physical similarities between little gray men and El Chupacabra lend credence to his suspicions.

Alien Rebel

Episode: Patient X, The Red & The BlackThese faceless men, who seal up their facial orifices to keep the "oilien" from contaminating them, use metallic wands to set people ablaze. The rebels attempt to foil the alien colonist's plans to take over Earth. One rebel is captured by the government and presumably killed by the alien bounty hunter.

Musings Alien

Episode: Musings of A CSMIn Frohike's version of the Cigarette Smoking Man's life history, this alien's space craft crash lands in West Virginia, shortly after the Roswell incident. Its pilot is critically injured. CSM and Deep Throat visit the warehouse where the alien is being kept on life support and shoot him.

Erlenmeyer Flask

Episode: The Erlenmeyer FlaskIn a top secret containment facility in Maryland, Scully steals an alien fetus, which has been used to develop an extraterrestrial virus. It is returned in exchange for Mulder, but at the expense of Deep Throat's life.