The X-Parasites are parasitical lifeforms indiginous to SR388. They are featured only in Metroid Fusion, and are the main enemies there, consisting of most of the creatures there as mimics of the original creatures, like the Hornoad. They could be considered Samus Aran's most dangerous foe, due to their danger being so great the Chozo created the Metroids, the galaxies greatest threat, to prey on them.
The X-parasites live upon SR388, and are suited to the planet, breeding and multiplying more in certain atmospheres of it. When the B.S.L. station was first beginning to be overrun with the X-parasites, they tried to convert Sector 1:Sector 1(B.S.L.) to have an environment more suited to them to breed, though Samus was able to destroy the X using the environmental stabilisers to do this, halting the change.
The X have the ability to infect other life-forms and multiply within them, either killing the host or controlling them. Once the life-form is dead, they can mimic its DNA to form a copy of the original life-form. The copy or host is usually more powerful than the original, amplified by the X to be more battle capable, such as giving Arachnos abilities not known to be available to its species. The X also gain the memories of the host, shared within the collective minds of the parasites. They cannot mimic machinery or anything inorganic, instead colouring themselves to resemble them, such was the case with Nightmares mechanical parts, and the parts of Samus's suit which were inorganic with the SA-X. The Ing race are similar to the X in that they too have these abilities, though are capable of possessing machinery and inorganic materials, and are incapable of being able to become the host again after seperation.
The only predator, and only threat, to the X-parasites are the Metroids, who were specifically created by the Chozo to hunt and prey upon them. The X could not mimic nor control the Metroids, taking away the X's strongest offensive and defensive ability. The Metroids were made by the Chozo due to their realisation that the X were too dangerous for the galaxy, and would mimic and destroy all life if they ever left their planet. The Metroids were able to successfully eradicate almost all the X-parasites upon SR388 after being placed there, leaving a small amount hiding away from their predators. They most likely hid within a cold region of the planet where the Metroids could not travel in, due to their vulnerability to cold. After Samus Aran, on the orders of the Galactic Federation and likely acting on her own hatred of the species too, eradicated the Metroid presence upon SR388, the X quickly repopulated the planet, their only predators destroyed.
The X-parasites have only made one appearance in the series so far.
 Metroid Fusion
At the start of Metroid Fusion, whilst Samus is on a mission by the Galactic Federation to escort researchers upon the planet SR388, she encounters an X-infected Hornoad. She easily destroys it, though the leftover X-parasite flies into Samus. She felt no immediate effects, and completed her mission. Upon leaving the B.S.L. however, she falls unconscious as the X multiply within her and her Power Suit, and her ship crashes into a nearby asteroid belt. Her ship ejects her from the cockpit, where she is collected and treated by doctors, though are unable to remove Samus's Power Suit to help her condition. They are forced to surgically remove the infected parts of her suit, though the X within her are still killing her. A scientist then suggests injecting her with Metroid DNA, which completely cures her from X-Parasite infection due to the Metroids being their predators. The X within her are destroyed, but the parasites within her quarantined suit begin to reform, creating the SA-X. It then escaped, and released the X upon the station.
Samus Aran is called back to the B.S.L. to investigate the happenings there, her changed cellular makeup allowing her to absorb any free-floating X-Parasites by touching them. She finds that the X have infected almost every life-form in the Main Sector, barring organisms they did not think were battle-worthy, such as the Etecoons and Dachoras. As Samus strove to retrieve her lost abilities from Data Rooms and Core-X, she releases many security locks, unfortunately permitting X the access to the other sectors, infecting all within.
The X show true intelligence by realising Samus is vulnerable to cold and several evolved to become an ice parasite, damaging Samus heavily when absorbed. They also destroy Data Rooms in order to stop Samus becoming fully battle-capable, and finally use the knowledge of an infected scientist to try and destroy the station to get rid of Samus, protecting the X on SR388 from her. To destroy her they also try to trick her with X disguised as power-ups and a Core-X imitating a Chozo Statue.
Ultimately the X are unable to stay Samus and all the X are destroyed by the games end, the SA-X even giving itself to Samus so she can destroy the Omega Metroid with the Ice Beam after them both being bested by it.