X Parasite

The X Parasites are a fictional species from the video game Metroid Fusion. In their natural form, the average X Parasite is a hovering, volleyball-sized amoeboid creature that glows yellow, green, or red. A mutated blue variety also exists. Sometimes several such parasites will combine into a larger, more powerful form, known as a Core-X. The X Parasite mutates easily in order to adapt to new environments. X Parasites have the unique ability to infect, copy the DNA of, and kill another creature, then use the stolen DNA to create a new body with the same form and behavior as the previous host. Whereas normal X Parasites can only imitate weaker hosts, Core-X can also imitate much stronger ones. The X are also capable of combining DNA from various hosts to create a new creature with the abilities of the individual host forms. The X Parasites can also download and process electronic data organically. When threatened, an X Parasite will revert to its amoeboid form and is extremely difficult to harm, except by attack from a Metroid. X Parasites reproduce by asexual division. Strictly speaking, the X are parasitoids (endoparasitoids to be exact) rather than parasites. In a parasitic relationship, the host is not generally killed, whereas in a parasitoid relationship, the host is usually killed after the full development of the other organism, which is the case with the X.

One of the startling revelations in Metroid Fusion is the apparent intelligence of the X. Although first believed to be merely instinct driven mindless organisms, the X exhibit remarkable adaptation and even plot various traps to defeat Samus. Either by duplicating sentience or intelligence from the host bodies they infected, or having always been intelligent, by the time of the game they have achieved intelligence at least of human level. The X, realizing Samus' new vulnerability to cold, actually mutate some parasites to be cold so that if Samus absorbs them or even touches them, she'll be severely hurt. A suit upgrade defends Samus from this problem, however the X also has the SA-X attempt many different ambushes in order to kill Samus. The X also mimic the dead corpse of a researcher in order to gain access to a particular computer terminal for their own purposes. The X even animated the dead corpse of Samus' longtime nemesis Ridley in order to exterminate her. When it is revealed that Federation Ships are on their way to retrieve the X, Samus knows that they will be no match for the X and will be used by the parasites to infect the entire universe. Indeed as Samus' A.I. companion Adam states, the intelligence of the X is nothing short of remarkable...and a little terrifying.

The X Parasites in the Metroid storyline
The X Parasites are such prolific breeders that on their home planet of SR388 they threatened to destroy the ecosystem. The Chozo stepped in and created the Metroids in order to keep the X Parasites' numbers in check. When series protagonist Samus Aran exterminated all of the Metroids on SR 388 in Metroid II, the X Parasites' population quickly made a comeback. They provided the main antagonist role in Metroid Fusion, particularly in the form of the SA-X, an X mimic of Samus Aran and her Power Suit. As the X continued to menace the station, the Federation took an interest in the X. They ordered Samus' computer to confine her until Federation ships arrive. Samus, knowing that the X would overwhelm them and spread through the universe, set the station on a collision course with SR-388. By doing so, all of the X on the station and the planet would be destroyed. The SA-X tried to stop her, but was defeated. It should be noted, though, that the SA-X comes into the melee between Samus and the Omega Metroid, and attempts to kill the creature (unsuccessfully). Samus absorbs the SA-X and defeats the Metroid, then escapes the doomed station. Whether the SA-X was attempting to save Samus or just kill the Omega Metroid is a mystery. However, seeing as the Metroids are the X Parasites natural enemy, and both Samus and the Omega Metroid had Metroid DNA, it is likely that the SA-X was simply trying to neutralize the greater threat (the Omega Metroid) first.