The X Parasites are the main enemy from Metroid Fusion. They are airborne, amoeboid parasitic creatures that can absorb a creatures DNA and become it. By absorbing the DNA, they also kill whatever creature they are copying.
The X come from SR388, the planet where Samus Aran exterminated the remaining Metroids. However, it turns out this act threw off the ecosystem of the planet. Metroids originally hunted the X so that they were not a threat to any life. With the Metroids gone, the X have grown rapidly, absorbing DNA and killing creatures. Because Samus' suit in Metroid Fusion is fused with Metroid DNA, Samus can absorb X Parasites to regain health.
The X also have the unique ability to come together and form a bigger creature. One notable X creature is SA-X, an X clone of Samus's suit. it can use all of Samus' abilities and originally used a famed ice beam super missile combo but when you get the special heat and cold resistant suit you just run away and they will not freeze you but when you get ice missiles you can freeze it is not known to have any purpose for it's actions Samus describes the x as mindless killing machines when she finds a secret metriod cloning project the sa-x destroys it technically the last metriod when her computer tells her there are 10 sa-x on board Samus goes to the operations room to set a course for SR388 she is confronted by the sa-x and defeats it sets the course and sees her ship is gone then the omega metriod slashes her leaving her with 1 hp then the sa-x faces it but weak from her battle with Samus it is defeated giving you ice beam also in setor 6 you also confront blue x which damage you on contact until you have the varia suit the x also infect Ridley who when he dies with out your help a core x can be seen floating away other bosses include nightmare a giant spider and they only appear in fusion (the x of course) there are beam core x which fire a beam at you witch you get two charge and wide witch are a boss on their own