Purity/The Valdez type aliens/The Black Cancer/The Black Oil/The Virus
The "Piper Maru" version of the black oil is able to enter a target human and take full control of it's host even gaining access to memories the human possesses.
If the organism feels threatened it is able to unleash a fatal blast of radiation from the body of the host. The alien can be forced to evacuate the host in two ways - by piercing the base of the host's neck with the alien weapon which is fatal to the host ("X-Files: The Game") or by injecting the host with the vaccine.
It is this form of the virus that the Syndicate believed they were dealing with when preparing the way for the extraterrestrial colonisation of the planet.
("...We believed the virus would simply
control us, that mass infection would make us a slave race." WMM: "X-Files: The Movie")
The "Tunguska" version of the Black Oil exists as a nest of microscopic worms which induces a Coma state in any human that it possesses.
The nest attaches itself to the Victim's Pineal gland and will evacuate its host if it becomes unable to support it. There is some evidence that the virus can exist within the host (The convalescent patients in "Tunguska/Terma")
without inducing a Coma state but that factor could be due to work with an experimental vaccine.
It is possible that this version of the virus is a weak strain (perhaps because of the cold climate) which is unable to fully mature.
The Movie version of the black oil is able to enter a host induce a Coma state and then grow into an Attack alien.
All versions of the black oil are apparently able to survive for a long time without a host. (the "Piper Maru" version was at the bottom of the ocean for fifty years, the "Tunguska" version survived in a meteorite since 1908 and the Movie version survived in a cave in Texas for 37,000 years.)
The "Piper Maru" and Tunguska" versions also seem to be able to be able to enter airtight spaces, the "Piper Maru" version entered an airtight submarine ("Piper Maru/Apocrypha") and exited a sealed Quarantine chamber ("X-Files: The Game"), while the "Tunguska" version entered a Quarantine suit ("Tunguska/Terma")
Apparently the virus will be spread by bees transporting the pollen of crops genetically altered to carry the virus. There are possibly two strains of bees - the bees in "Zero Sum" seemed more likely to swarm and attack even when not provoked than the bees seen in "Herrenvolk" and the movie.
Attack Alien
The Attack Alien gestates from the body of a human infected with Purity. It will attack anything that it sees as a threat to its survival and when injured the Black Oil is released from its wounds. After some time and in hot conditions the attack alien will shed its skin and transform into a grey alien.
Grey Alien
The Grey is the most advanced form of the colonising alien, they bleed green blood ("Nisei" and "Musings of a Cigarette Smoking Man") and conduct abductions in collaboration with the Syndicate in the pursuit of creating an alien/human hybrid. They may (or may not see section on shapeshifters) be the same race as the shapeshifters.
Shapeshifters/Morphs
The shapeshifters are a race of extraterrestrials with the ability to change their appearance into the likeness of somebody else. They have superhuman strength and may have the ability to heal wounds ("Talitha Cumi/Herrenvolk"). When injured they bleed a toxic green fluid. The only way to kill a shapeshifter is to pierce the base of their neck with the alien weapon.
Certain signs indicate that the shapeshifters and the greys are the same race. They both bleed green blood, the black oil (an early growth stage of the grey) can be forced to evacuate its host by piercing the base of the host's neck with the alien weapon (the same method is used to dispatch the shapeshifters) and in "The Unnatural" (which may be unreliable) we saw a grey 'morph' into a shapeshifter.
Faceless Aliens/Rebels/The Resistance
It is unclear whether the faceless aliens are a rebel shapeshifter faction or an entirely different race. They have the ability to shapeshift, have superhuman strength and bleed a green fluid. They have mutilated themselves to prevent infection by the black oil. They can be killed by piercing the base of their neck with the alien weapon.
Human Replicants
Created in order to aid in the repopulation of the planet after the extraterrestrial colonisation, the 'replicants' are described as being a new type of alien. Like the attack alien the organism originates as a virus which develops inside a host body. Once the replicant has fully developed it sloughs off the skin of its host and emerges having taken the form of the host body. Unlike other forms of colonising alien, the replicants bleed non-toxic red blood but like their brethren they do have phenomenal strength and are impervious to harm.
They are recognisable by a bump on the neck, which is apparently a metallic vertebrae which will replicate and eventually replace the entire body of the replicant if it becomes too badly damaged.
Radioactive Alien/The Fallen Angel
The radioactive alien is an invisible alien that is only observable as a wavy rippling effect. It can emit fatal (and nonfatal) bursts of radiation and can move at a speed of 7mph. The military is anxious to destroy it but this is more likely due to Security Council Resolution 1013 (see "Musings of a Cigarette Smoking Man" and "EBE") than any involvement with the plans for colonisation.
Ice Worms
The ice worms came to Earth on a meteorite which crashed into Icy Cape, Alaska over 250,000 years ago. Since then they have lain dormant under the ice. They thrive in an ammonia based environment and are capable of entering a host and causing their brain to release acetycholine - a hormone which produces aggressive behaviour. Two different worms will attack and kill each other, thus neutralising infection.
Alien Spiders
It is unclear whether or not the spiders are extraterrestrial in origin. During the cold war they were grafted into the throat's of test subjects (xenotransplantation). They have the ability to emerge from their host's mouth and remove their victim's soft tissue. They influence the behaviour of their host so that the host uses them to kill, however the host remains aware of who he is.
The Kindred/Genderbender aliens
The Kindred are an 'Amish' like group who shun modern technology and live an isolated life. They have the ability to change their appearance in a similar fashion to the shapeshifters but only to a single male and a single female form. The fact that their pheromones contain human DNA seems to suggest that they are some form of hybrid, however they have existed since at least the 1930s before the hybridisation project (Purity Control) began. It is possible therefore that they are an early trial run at hybridisation by the aliens themselves rather than the Syndicate.
Lord Kinbote/Behemoth from the planet 'Haryhausen'
Lord Kinbote is not strictly an alien, he lives in the molten core of the Earth where he engages in sex orgies. It is unclear whether he really exists or not. He could plausibly be a USAF creation, a hypnotic suggestion or a genuine entity.
Mars Alien/Space Ghost
This alien resembles the 'face' on Mars. It is apparently able to possess and influence the actions of a host, although the host does regain some control over his actions. It is seemingly opposed to man's presence in Space and has worked to sabotage several NASA projects.