This 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.
Episodes: Colony, End Game, Talitha Cumi, Herrenvolk
This 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.
Scientists 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.
This member of The Kindred, a remote religious community, picks up strangers in bars and then muders them through hormonal overload. Bro. Martin is able to take the shape of either a male or female. Intent on protecting their secrets, the other members of The Kindred take the killer and disappear, leaving only a mysterious crop circle behind.
Released 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."