The creature took on the host's memories, appearance, abilities, everything. It sucks everything dry. Utterly terrifying. Can you not see that if a mimic such as this were to infiltrate a population, it could replicate almost indefinitely! With evil taking the form of an ally, the galaxy could be overthrown. It is truly the worst kind of evil... it has no name, but... we call it X.
~ Old Bird on the X Parasites.
The X Parasites (sometimes spelled as X-Parasites) are the main antagonists in the videogame Metroid Fusion. They are hostile parasites that were the Metroid's prey, but after the Metroids became near extinct, the X Parasites took over a space station where Metroids were being researched, and caused enough threats to nearly wipe out the universe if it was not for Samus Aran.
They have the ability to take the form of anything they make contact with. Doing so, the X Parasites created SA-X, B.O.X., Nightmare, Neo-Ridley, and so much more.
They are also the overarching antagonists of the entire Metroid series, as their existence is the reason that the Chozo created the titular creatures in the first place.
The appearance of an X Parasite is that of a floating gelatinous organism. The structure of their bodies allow them to slip through any opening of a surface, rather it be the pores of an organism or crevice of any surface. They have the ability to multiply via asexual reproduction. The X Parasites absorb into their host through open surfaces, and target the central nervous system, eating away at it and duplicating until the host's death. Upon the host's death, the X Parasites are capable of imitating their host by replicating the host's DNA, also granting them the memories and behaviors of said host.
The most basic and common type of X Parasite. When absorbed, they restore 10 units of energy. A single Yellow X was responsible for infecting Samus and nearly killed her if it was not for the Metroid vaccine.
Green X appear after Samus regains her Missile Launcher. Similar to Yellow X, the Green X can be found anywhere on the BSL station and excrete from most enemies. They restore two Missiles when absorbed.
The rarest out of the three common X Parasites. Normally appearing from Gadora-X and other strong enemies, they restore 500 units of Energy, 10 units of Power Bomb Ammo, and 50 units of Missile Ammo upon absorption.
The Blue X (or Cold X in Japanese) are a special type of X Parasite that have adapted themselves to the cold temperature of Sector 5 of the BSL station. Apparently, the X Parasites deliberately forced their evolution to stop Samus from exploring the rest of the space station, explaining why Blue X have invaded Sector 6 as well. Until Samus obtains the Varia Suit, Blue X will chase down after Samus and, due to the Metroid vaccine, absorb themselves into her in order to damage her, although Samus can shoot at them to temporarily stun them. After Samus regains her Varia Suit, Blue X will try one last time to contact her, but after this, any future Blue X will learn from this mistake and flee from Samus instead. Blue X will both inflict and restore 30 units of energy. Because of their added cold immunity, they are much larger than regular X Parasite and are oddly unable to infect and possess a host, presumably due to their size or cold body temperature.
Core-X are the largest and strongest members of the X Parasites. They have incorporated themselves into either an important ability from the DNA of their victims or one of Samus' lost power-ups. They take the form of a boss and, upon defeat, reveal their natural form. They will then pursue Samus until she or it is defeated and its nucleus is exposed for Samus to absorb. The Core-X were originally regular X Parasites, but the source of their prey made them stronger and allowed them to produce X Parasites at an alarming rate. Using the Screw Attack or shooting at them with a beam or missiles will make Yellow and Green X fly out of it.
These Core-X are protected by a strong outside shell that have to be broken with Missiles until their nucleus can be obtainable.
These Core-X hold a power-up beam that can only be damaged when they expose their nucleus-like eye from underneath their Missile immune exterior. Their eyes only open when they are about to shoot out a laser, meaning Samus should be cautious about timing her shots to avoid theirs.
An X Parasite appears in Super Smash Bros. Ultimate as an attack-type primary spirit. It can be obtained by purchasing it from Beetle's Tent for 500 SP or by defeating it in a spirit battle, which is against ten yellow Kirbys on the Lylat Cruise stage. The spirit can be enhanced to Nightmare upon reaching level 99.
In early beta trailers for Metroid Fusion, X Parasites were recolored sprites of the Boyon enemies from Super Metroid.
The X Parasites have similarities to The Thing as they both can infect a host and copy their physical appearance and abilities.