https://static.tvtropes.org/pmwiki/pub/images/x_parasite.png

"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

Parasites that evolved on SR388, where they began to greatly endanger the planet by absorbing the biomass of other organisms, copying the victims, and throwing the ecosystem out of balance. These creatures were such a massive threat, the Chozo of the Thoha Tribe intervened by giving them a predator: The Metroid. Metroids were their main predator, until all of the Metroids were wiped out on SR388, allowing the X to make a recovery in their population and return in Metroid Fusion.

BEWARE OF SPOILERS

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In General

Achilles' Heel: Metroids were made specifically to kill X, and they're really good at it. Even just a single touch from a Metroid is enough to kill and absorb most X. Any other method of killing them requires an absurd amount of overkill that isn't remotely practical given how numerous they are. Thankfully for the galaxy at large, Samus' Metroid DNA is enough for her to exploit this weakness, too.
Always Chaotic Evil: All X Parasites are extremely dangerous, kill indiscriminately, and, as detailed under It Can Think below, are sapient. However, Dread brings this trope into question, especially with Quiet Robe X's sacrifice.
Ambiguously Evil: At first, they are assumed to be Always Chaotic Evil, Quiet Robe's doppelganger displays odd behavior at the end of Dread, willingly allowing itself to be absorbed by Samus, presumably to "reverse" her transformation into a Metroid.
Aquatic Mook: They are more than up to the task of mimicking aquatic creatures^note to fight you when you're in the underwater sections of the B.S.L station. They will also combine Space Pirate DNA with Skultera DNA to make Aqua Zebesians that have no trouble swimming right up to you and taking chunks out of your health.
Artificial Brilliance: Mid-way through Fusion you'll encounter some blue X that are very cold. This is at a point in the game where you are very vulnerable to cold and will have to avoid them like the literal plague they are in a nasty inversion of Power-Ups. Once you collect a power-up that allows to absorb them safely though, the blue X will, at first, attempt to swarm you like before. But soon after they flee on sight after seeing that their tactic no longer works. This entire exchange is the first hint that the X may be more sentient than first thought.
The Assimilator: They infect creatures and consume them on a cellular level, absorbing their DNA and using it for their own ends. They retain their hosts' knowledge and abilities for their own use.
Astonishingly Appropriate Appearance: Their name may be derived from Let X Be the Unknown, but the basic form of the X has throbbing pseudopods that make them look like bulky, gelatinous versions of the letter X.
Back from the Dead: They're shown to be capable of assimilating DNA from corpses as well as living creatures. As Fusion takes place after Other M, the Space Pirates are all but wiped out, Ridley was killed off for real on Zebes, while his clone and Nightmare were taken down in Other M. The X infected the frozen samples and corpses sent to B.S.L for study and began mimicking them, bringing them back for another go against Samus. In Dread, the X infest the dead body of Quiet Robe and use his knowledge to reactivate the E.M.M.Is he disabled earlier.
Better to Die than Be Killed: The X has a pretty high chance to run after their corporeal form is destroyed, but they'll usually stay and try to fight Samus to the death.
Big Bad: They are collectively the main antagonistic force of Metroid Fusion. The SA-X, an X Parasite with Samus' DNA, is the most recurrent and dangerous of their number and caused the explosion that released its brethren around the B.S.L station and causes most of the deliberate damage that Samus has to circumvent.
Big Bad Ensemble: In Dread, they serve as an equal, unrelated threat to Raven Beak: While Raven Beak is the more personal of the two antagonists and is after Samus specifically, the X are every bit as dangerous after they break out, and both threats need to be dealt with for the galaxy to be safe.
Blank White Eyes: As shown by the SA-X and later Chozo mimics, a common Glamour Failure of X mimickry is pure white eyes without irises or pupils.
Blob Monster: The appearance of the X in their true form, though unlike most they fly. Dread furthers this by having all X-mimics be formed from Ominous Obsidian Ooze produced by the parasite core.
Body Horror: Through the process of infecting hosts, the X either take over the body or replicate it after consuming it, typically while mutating the original form into something much worse (explained to be the result of mixing and matching DNA from multiple victims). Samus doesn't even have to be fully infected for this trope to come into play for her.
Bootstrapped Leitmotif: They borrow the "Tension Before A Confrontation" pre-boss tune from Fusion for their appearance in Dread. Quiet Robe X, Escue, Experiment No. Z-57, and Raven Beak X all have the song mixed into their respective themes.
Chest Monster: In addition to Elephant Bird, an X mimics a Missile Tank in Sector 6, while another masquerades as an Energy Tank in Sector 5. When approached, eyes and bat-like wings appear on the tank, and it flies into the air.
Clipped-Wing Angel: Once the body they are using is destroyed they will revert to the basic X form, which Samus (thanks to her new Metroid biology) can literally eat on contact.
Color-Coded for Your Convenience: Done to indicate the effect different X varieties have when Samus consumes them. Yellow X restore health. Green X restore missiles. Red X restore a lot of both. Blue X initially hurt Samus but restore a lot of health after she gets the Varia Suit. Dread introduces an orange X that restores Power Bombs.
The Dreaded: They are widely considered one of the most dangerous organisms in the galaxy. In the manga and Chozo Memories of Samus Returns, the Chozo are quite frightened when describing what the X do, and they were so desperate to check the spread of the parasites that they created another dangerous species, the Metroids, just to keep their population in check. In Fusion, Samus is so desperate to keep them from spreading across the galaxy that she briefly considers giving up her own life if that meant taking them out as well. And Dread opens with the Federation sending seven Nigh-Invulnerable robots to ZDR, followed by Samus when said robots disappear, over the mere possibility that the X have survived.
The End of the World as We Know It: This is more or less what happened when the X made a comeback on SR-388. Without a predator to stop them, the X devoured all life on the planet. It's also what would have happened elsewhere had the Galactic Federation gone through with their plans to capture the SA-X. After escaping Elun in Dread, they overrun ZDR within hours of getting loose.
Evil Only Has to Win Once: The main threat regarding the X and why the Chozo were so desperate to deal with them, to the point that they created a bioweapon almost as powerful as the X; the outbreaks on SR388, the BSL research station, and ZDR were only the start of what the X wanted. The X are, on a genetic level, predisposed towards propagating their species and ensuring their own survival. They have no other motive, and nothing short of a Metroid can truly hamper them for long, making any X outbreak a living Grey Goo scenario. All the X need to more or less completely consume everything in the universe - and what they intentionally set out to get in the BSL research station and on ZDR - is a form possessing memories that would allow the X to spread off-world and throughout the galaxy, and a spacefaring vessel they could then use to exploit those memories.
Evil Versus Evil:
- They flat out hate Metroids and will ignore Samus just to attack their natural enemies.
- Dread put them against the Mawkin Tribe on ZDR, in the backstory, wiping out all but Raven Beak before he sealed them away which preventing him from enacting his own evil plan earlier.
Expy: Similar to how the Metroids are a Xenomorph Xerox, the X are an Expy of The Thing. Both infect hosts and assimilate them on a cellular level, and both imitate their previous victims for their own use.
Gameplay and Story Integration: Samus was injected with a serum the made her part Metroid. Metroids are able to devour X at the cellular level. The X you encounter in the game that are in their natural forms serve as your pickups and you are supposed to absorb larger Core-X to regain lost abilities.
Genetic Memory: They retain the intelligence and memories of what they infect and assimilate.
Geo Effects: They are smart enough to make use of them, particularly their use of cold areas to ward off Metroids.
Godzilla Threshold: The existence of the X and the Federation's desire to study and use them is one of the biggest thresholds in the entire series. As mentioned, they were the entire reason the Metroids exist, which themselves are an incredibly dangerous threat to the galaxy already, as they were created by the Chozo to combat the seemingly Nigh-Invulnerable threat. Samus considers detonating the BSL research station, even at the cost of her own life, to be worth it to destroy the X onboard. Adam's response is that this doesn't go far enough and that the station should be rammed into the planet below as it explodes in order to be well and truly rid of the X.
Greater-Scope Villain: For the entire series, especially Metroid II/Samus Returns, as the Metroids were created exactly to counter their infestation of SR388. And the X in general are much more dangerous to life due to their higher intelligence and being harder to fight off. Everything in the series can be traced back to the X in some form or fashion. Fittingly enough they return and are well and truly (most likely) eliminated in Dread, the Grand Finale of the "Samus and the Metroids" storyline that spans the entire series.
Humanoid Abomination: Early in the game Samus encounters gelatinous humanoid blobs wearing lab coats, the result of X infecting slime creatures before infecting the research staff and muddling the two sets of DNA. The X end up refining the process eventually, to the point where a Core-X perfectly mimics a scientist in order to set Sector 3 on the fritz. There's also the SA-X, which wears Samus's old Varia Suit with all of its armaments intact.
Informed Attribute: While the manga did illustrate that the native life on SR388 fled from X on instinct and were still getting wiped out despite this adaption, it failed to demonstrate how they were "evil" or how anyone could conceivably use them as a tool for infiltrating the Federation (the game makes it clear their only drives are eat, reproduce and eliminate any perceived threats. It then stated anyone who thought they could be controlled was kidding themselves). However, Artificial Brilliance and It Can Think does show they are smarter than they look.
Intangible Man: Yes, they can go intangible and ignore most physical barriers. However, it is implied by the powerbomb that large enough explosions can still affect them, and it's stated by Adam that big enough ones can kill them.
It Can Think: At first, they seem like a mindless species of parasites running purely on instinct. Over the course of the game, you begin to learn that they're ruthless, intelligent, and capable of acting against instinct for their own ends.
It's the Only Way to Be Sure: Once the X have been broken quarantine on an ecosystem, it can be safely assumed that the entire planet is done for. Due to its ability to mimic anything, down to their appearance and memories, there is no way to tell what is or isn't an X, so anything and everything within its reach needs to destroyed for the safety of the galaxy. Since this spread can occur within hours, that basically means anyone who isn't separated by extreme distances or hermetically-sealed environments cannot be allowed to escape offworld, as the whole mess starts all over again if even a single X ever finds its way into space.
Kill and Replace: The X had already done this to the fauna of SR388 prior to the events of Fusion, allowing them to infiltrate the orbiting research station when the researchers collected X mimicking biological samples such as Hornoads from the planet's surface. This also spelled the doom of the Mawkin Tribe, when an X Parasite impersonated one of Raven Beak's soldiers and started a new outbreak on ZDR.
Kill It with Ice: They're well-aware of a Metroid's greatest weakness and they attempt to use it against Samus whenever they can. In Fusion, several of them hang out in the arctic sector of the station and make themselves icy-cold, which works until Samus gets the Varia Suit. In Dread, they clog-up the thermal vent from Cataris, which results in Artaria freezing over completely. It doesn't work at all since Samus has the Gravity Suit by the time they do it, though it does slow Samus down due to freezing-over several doorways. And they were working on the freezing process almost as soon as Samus released them - while Samus was collecting the Gravity Suit in Burenia, they were already gumming up the thermal vent. It was simply luck on Samus' part she found a counter to the X's plan just as they were ready to implement it.
LEGO Genetics: Not only can they mimic the DNA of their prey, but they can combine, mutate, and alter that DNA as they see fit to produce ever more dangerous forms.
Let X Be the Unknown: In reference to their nondescript nature and appearance without copying a different creature.
Logical Weakness: They can copy anything organic, but they can't replicate machines (though machines with organic parts are fair game). The E.M.M.I. were dispatched with this in mind in Dread.
Mix-and-Match Critters: The X have displayed the ability to absorb the DNA of multiple species and then mash them together to create hybrids with traits of both species. In Fusion, this is best exemplified by the Aqua Zebesian, which is a Zebesian with the tail and fins of a Skultera. It's taken farther in Dread, which features many more hybrids, including Bigkran (Muzby and Vulkran), Gooshocker (Gooplot and Ground Shocker), Nailugger (Nailong and Slaaga), Quetshocker (Quetzoa and Ground Shocker), Rodomithon (Rodotuk and Obsydomithon), Spitclawk (Spittail and Sclawk), and Yamplot (Yampa and Gooplot).
Nigh-Invulnerable:
- What truly makes them so dangerous. It's one thing to kill the creature an X poses as - killing the X Parasite left over is practically impossible, and anything that could kill whatever the X was mimicking simply makes an even better target for X possession. A combination of intangibility and natural immunity to most munitions means even armies of soldiers are helpless before them - which is why ADAM and Samus do everything they can in Fusion to prevent any armed forces or researchers from coming into contact with the X. An entire species had to be specifically bioengineered by the Chozo to overcome the X, because no Chozodian mysticism, machine, or weapon could pose a threat to them.
- Beyond a Metroid, the only sources of power that have been shown capable of killing an X include the heat of a planet's mantle (and maybe not even that), a planet-destroying explosion, and the Hyper Beam, a weapon so powerful it can tear apart mountains and reduce several hundred tons of fortified steel capable of resisting Power Bomb explosions to dust in less than a second of sustained fire. And even then, the X can still mimic forms capable of surviving molten heat and could even assume forms that could survive multiple seconds of sustained Hyper Beam fire, the latter seen with Raven Beak X.
Non-Malicious Monster: Despite being described as "evil" by the Chozo, the X act much closer to a typical animal species, always looking for the self-preservation of its kind first and foremost, thus they expand their populations and take down any potential threats to further its chances of survival (some X will even sacrifice themselves to ensure the safety of bigger populations). It's just that the methods it employs to do so are considered horrifyingly destructive and inhumane for any sentient being and, if left unchecked, all universal life would end up in conceivable danger, but because the X is an alien parasitic lifeform akin to a giant virus or bacteria, it just plays by the evolutionary rules and tools it was given. The fact that it might be sapient and capable of emulating a victim's memories and personality, with the latter ability possibly influencing its decision-making process, further muddies things up.
Not Quite Dead: If the body they are mimicking takes enough damage, the X can still survive. If Samus doesn't absorb them quickly (she has some time to do so as the X is left stunned for a few seconds and doesn't move that fast) the X mass will run a bit away and reform a new mimic body or try to join another mimic to form a stronger one.
Not So Extinct: Samus seemingly wiped them out when she crashed the B.S.L station into SR388. But the plot of Metroid Dread is set off when a video transmission from planet ZDR shows a X Parasite still existing, causing the Galactic Federation to send the E.M.M.I. to investigate their mysterious survival. It is later revealed that when the Mawkin Tribe of Chozo came to SR388 to exterminate the Thoha Tribe, a X Parasite disguised itself as a Mawkin warrior, leading to an outbreak that wiped out everyone but Raven Beak who sealed the X Parasites away in ZDR, allowing them to outlive the original colony in SR-388. Though given how ZDR is blown to hell in typical Metroid fashion at the end of the game, it's safe to say that they’re gone for good this time.
Ominous Obsidian Ooze: In Dread, their mimickery produces black, tar-like goop that spreads out and hardens to form the parasite's desired shape.
Ominous Visual Glitch: Their shapeshifting in Fusion takes the form of pixellating into an indistinct blob of color, before coming back into focus with a new form. Bosses, as they near death, start losing visual clarity as the Core X inside struggles to retain its shape.
Poison Mushroom: Unlike most X Parasites, the blue X will injure Samus if she absorbs them because their ice cold bodies exploit her Metroid DNA's weakness to cold temperatures. They will even chase her down in order to freeze her to death. Once Samus gets the Varia Suit, they can be safely absorbed like any other X.
Possessing a Dead Body: They don't need their targets to be alive in order to absorb their memories and abilities, as seen with Neo-Ridley and Quiet Robe.
Power Parasite: Since Power Suits are Organic Technology, X have repeatedly shown the ability to steal and use Chozo powerups for themselves, forcing Samus to absorb them in turn to get the ability for good. Even putting the SA-X aside, most bosses in Fusion (as well as a few bosses in Dread) are shown actively incorporating whatever power they absorbed into their new body, sometimes in a logical way (the one who uses the Morph Ball turns into an Arachnus, which is already capable of curling up and rolling around), sometimes turning into aberrations like a Giant Spider doing backflips even without limbs (the Yakuza, having taken the Space Jump) or a hermit crab summoning clean geometrical patterns made of explosions (the Golzuna, who holds the Cross Bomb).
Power-Up: Interestingly, while they're the main threat of Fusion they're also your main source of energy.
Punch! Punch! Punch! Uh Oh...: The blue X that have cold as a part of them exploit Samus' initial vulnerability to them as a part of her new Metroid DNA's inherent drawbacks. Then after she gets the ability to handle said cold via a restored Varia Suit function, they initially try to rush her like they had been before — only to get rapidly absorbed, realize shortly after it is not working as intended, and start panicking and fleeing again afterwards.
Shapeshifting Failure: More intelligent beings like humans or Chozo give the X problems when trying to mimic them, resulting in a blob-zombie loosely resembling the infected. The more problematic X are the ones that get it right; one infected Mawkin Chozo brought an end to the Mawkin tribe and eventually all of Planet ZDR, and an infected human scientist nearly killed Samus by trying to self-destruct the B.S.L station. And that's not even getting started on the SA-X.
The Soulless: Samus muses that the X can mimic the knowledge, powers, memories, and abilities of their victims perfectly, but they cannot copy the soul. This is supported by the X driven by a mindless desire to consume everything despite being fully capable of planning and going against instinct to solve problems. Though this does implicitly get called into question by the actions of Quiet Robe at the end of Dread.
Spanner in the Works: One of the Mawkin warriors on SR388 was assimilated by an X Parasite right before they left the planet. As soon as the Mawkin returned to their homeworld of ZDR, the X proceeded to infect the entire tribe, which ended up delaying Raven Beak’s plan to harness the Metroids until he could contain the X outbreak. By the time he had done so, Samus had wiped out the entire Metroid colony on SR388, forcing Raven Beak to pursue more elaborate means to acquire his Metroid army.
Spikes of Villainy: X seem to add them to a lot of the things they mimic, such as the modern appearance of an X-infected Hornoad from Samus Returns, adding more thorns to the legs of X-infected Dessgeegas, or the Goliathes, the Muzbies' stronger variants.
There Is Another: The X are so destructive that SR388 and the B.S.L station were detonated entirely just to get rid of them before they could spread. The events of Dread kick off with a video transmission of an X Parasite on Planet ZDR being sent to the Galactic Federation, which causes them to send seven of the E.M.M.I. units, and later Samus, to investigate the planet. It turns out that the X Parasites had gotten off of SR388 a long time ago, hitching a ride with the Mawkin Chozo returning home to ZDR. They wiped out the Mawkin tribe and were sealed away by Raven Beak before they could infect him.
Token Heroic Orc: Quiet Robe's X, after an initial bout of reawakening the E.M.M.I. to attack Samus due to their pre-existing malevolence, distinctly becomes the only X to actively aid Samus without any hint of ambiguity by sacrificing itself to restore her unstable genetics.
Ultimate Life Form: The X is probably the closest naturally-occurring form of this in the Metroid universe. It can infect nearly anything with even some biological components, assimilate it, and then mimic it perfectly via the absorbed knowledge of the victim or mess with its genetics to mutate it into a more "useful" form to survive in any environment. And even if its copied form is destroyed, the original X can escape and reform it with time. Then there's the fact there's seemingly no limit to how many different DNA samples it can obtain, allowing it to combine them at its leisure, and they can both reproduce asexually (with each copy having all the stored DNA) as well as combine with other X to get stronger and share DNA. Short of flat-out blowing up the environments they infect, the only way the Chozo could effectively combat the X was to bio-engineer an even better Ultimate Life Form to act as a predator.
Viler New Villain: The Metroids were feared and dangerous, but what the X do is even more horrific and their ability to infect, rapidly spread, and intelligence makes them far more dangerous to the point the Metroids had to be created to contain them. And while Metroids have shown the potential to be or used for good, X have been shown as inherently evil and destructive such that even those with the intelligence to act against said instincts only did so to further their spread.
The Virus: Though not literally viruses, you do fight both infected victims and mimics made by the X using their hosts' DNA.
Voluntary Shapeshifter: They can shift into the form of any of their previous hosts, often mutated to be more deadly.

Core-X

When an X Parasite takes on a powerful enough form, they become a Core-X. These enormous X are covered with a thick thorny membrane that shields them from Samus and must be destroyed with Missiles. Some Core-X, officially designated as Beam Core-X, have adopted beam weapons of their own and fire upon Samus. Once defeated, Samus can absorb a Core-X to gain a new ability.

Boss Arena Recovery: Core-X spawn regular X during their battle, including whenever they are hit by a weapon other than Samus's missile. However, since these smaller X are incapable of actually fighting against Samus and will instead try to flee, this averts Flunky Boss and actually serves as means for Samus to recover energy and missiles.
Clipped-Wing Angel: When bosses take too much damage, they revert to their Core-X form, losing the unique capabilities of their mimic body but gaining speed, flight, intangibility, and brief periods of invulnerability after each hit, but its clearly just one last desperate struggle with no attacks besides ramming Samus or shooting a simple projectile. That being said, the final core forms can be hairpulling especially after the hardest boss fights, such as Nightmare and the SA-X, as by that point it's very likely Samus is one step away from death.
Last Ditch Move: Once the body they are using is destroyed they can still hang around. When this happens to bosses, the remaining X can form an armored core to make a Last Stand. They can't do much besides ramming Samus and a few can shoot very telegraphed projectiles, but this last form can be challenging if the prior boss fight left Samus badly injured, which is particularly problematic in the second, harder half of the game.
Mercy Invincibility: After a Core-X is hit with a missile, it flashes white for a few seconds. During this time, it is invulnerable.
Nice Job Fixing It, Villain: Throughout Fusion, Core-X absorb upgrades meant for Samus from Data Rooms, forcing her to absorb those abilities from the X instead. Late in the game, however, it is revealed that the Federation was actively holding back certain abilities like the Plasma Beam and the Diffusion Missiles so that she wouldn't be able to destroy the SA-X, which they wanted to capture for bioweapons research instead. So by stealing these upgrades and "giving" them to Samus directly, the X make it where she can more easily disobey the Federation's orders and obliterate the X on both the B.S.L station and SR388.
Spikes of Villainy: The Core-X's outer membrane is covered with spikes.
Tactical Suicide Boss: Beam Core-X must open their protective membrane to shoot Samus with their beams, leaving themselves vulnerable in the process.

Biological Space Laboratories

SA-X (spoilers)

https://static.tvtropes.org/pmwiki/pub/images/nightmare_fuel_metroid_2.png

^{Click here to see its mutated form}

Debut: Metroid Fusion

"The SA-X is mimicking you at full power. You can't face it. If you see the SA-X, just run."

— Adam AI

An X mimicking Samus herself after forming from the surgically removed fragments of her armor.

Artificial Brilliance: During the final showdown with it, the SA-X will notice your fighting patterns and adapt to them. If you try to evade its attack often by jumping over it, it will soon answer with a Screw Attack right in that direction, hanging from ledges will lead to it shooting you down with missiles so that the tremor drop you down. If you keep evading the Ice Beam attacks with the Screw Attack invulnerability to projectiles, it will start to dive you with its own Screw Attacks, which deal high collision damage. Albeit, two colliding Screw Attacks will also deal damage to it and it reacts to being damaged the same way each time. Once one finds comfortable way of doing damage and avoiding the inevitable counter attack the SA-X will quickly crumble.
Artificial Stupidity: Despite the fact that the SA-X is the most powerful enemy in all of Fusion, and despite the X's ability to copy any organic body part, it apparently lacks a brain. The SA-X has absolutely horrid looping programming, to the point most can be exploited for humor. Power Bombs in particular seem to completely throw its programming into an infinite loop when Samus isn't in a room needing the Power Bomb to advance. This can be exploited in the boss fight with the SA-X. There is a ramp above the recharge station in the room where you must send the space station into SR388. Continuously charging and shooting SA-X will cause its AI to instinctively charge at you with a Screw Attack in an infinite loop as long as you know what to do.
Arm Cannon: As part of its body.
Attack! Attack! Attack!: While it has Samus' weapons and dexterity, it's otherwise a completely feral creature. All of its time is spent idly roaming the halls of the station in search of prey, and when it encounters anything objectionable its only strategy is to charge in and start blasting. Usually this is enough, but two SA-X instances meet their ends when their aggression leads them to pick suicidal fights with Metroids.
Big Bad: It was the one responsible for the disaster and for releasing all of the captured X into the station. It is also responsible for all of the tactical and deliberate damage done to the station that hindered Samus' progress and blocked off necessary areas, as well as being the more personal and dangerous opponent in the game.
Boring, but Practical: It only has 3 attacks, the Ice Beam, Screw Attack and a rare Super Missile (which it only fires after hitting you with the Ice Beam). However, all of them hit like a truck, are very fast and the SA-X can deploy them in a plethora of adapting patterns, making it all the same an extremely challenging opponent, since your charged beam and Screw Attack are the only effective ways of damaging it. Also true of its Armored Core form, which can only chase you and shoot at you with the Ice Beam. Doesn't do much else, but its hard to hit its vulnerable eye with a missile, and its weapon is still the Ice Beam. Like all Core-X, the Plasma Beam will produce numerous X for which to regain health and ammo, and as a Beam Core-X it moves at a glacial pace, so if the player can survive for half a minute the challenge amounts to successfully aiming and timing diffusion missiles.
Clipped-Wing Angel: When it takes too much damage it transforms into a gigantic hulking monster. It packs a serious punch but can be destroyed in only three charged shots and its laughably easy to evade its only attack, which is jumping around.
- However, after this form goes down, it turns into the Beam Core-X, which can chase down Samus and use the Ice Beam to attack, as well as being immune to Samus's weapons unless she targets the eye while it prepares to attack. In this final desperate form, this trope is downplayed, as while it is definitely less difficult than the fight while the SA-X had its stable Power Suit form, the arena where the SA-X is fought makes the timing of hitting the eye when it opens difficult^note not to mention it is still the fully powered Ice Beam, which instantly rips off 2 entire health tanks. After the brutal first part of the fight, the core portion can be especially stressful as Samus is likely low at health already and landing a hit on it demands her to expose herself to a direct Ice Beam.
Clothing Appendage: Its "armor" is integrated into its body, though unlike Dark Samus, it's also copying Samus' human form underneath (or at least her face).
Contrasting Sequel Antagonist: To Mother Brain. Both are artificial intelligences and both want to kill Samus. But while Mother Brain herself is the de-facto leader of the Space Pirates, SA-X isn’t the truly a leader of the X Parasites and Mother Brain is, well, a brain, SA-X is a copy of Samus at least in her suit.
Doppelgänger: Of Samus.
Dragon-in-Chief: It doesn't seem to truly lead the X but it serves as the primary antagonist due to its intelligence and power.
Double Jump: It has the Space Jump just like Samus. Adding the Screw Attack which does a lot of damage, so trying to run away from it in the air is a pretty bad idea, especially since its faster than you too.
The Dreaded: Samus and the player spend most of Fusion hiding from it and the rest fleeing in terror. It's only in the final stretch of the game that Samus becomes strong enough to stand even a ghost of a chance against it and even then the Final Battle with it is widely regarded as one of the toughest fights in the entire series.
Enemy Mine: Rather than finish off Samus when she's on the verge of death at the hands of the Omega Metroid, it instead tries to kil the Omega Metroid, though it fails. A charitable interpretation is that it considered the Omega a Godzilla Threshold that needed to die, and let Samus absorb its essence to finish the job.
Evil Versus Evil: It attempts to kill all the Metroid clones in the restricted labs in a violent fury, and then decides to rush the Omega Metroid at the very end because of the X's excessive hatred for the species. Neither attempt works out too well.
Final Boss: A three-part showdown goes down with the SA-X at the climax of the game as Samus decides to perform a Final Solution on the X, with the SA-X showing up just as Samus is trying to go into the Operations Room to crash to Station into SR 388. An Omega Metroid shows up later, however, its just a Post-Final Boss as the fight is very easy, even with the time limit.
Foil: The SA-X is quite literally a copy of Samus, with her darker traits set exaggerated. That nigh-unstoppable monster, seemingly mute and emotionless, blasting its way through obstacles in an attempt to wipe its target out - now Samus knows how the genocided Metroids of SR-388 felt.
Glass Cannon: Compared to most other final bosses in the series such as Mother Brain, Metroid Prime, Dark Samus, The Metroid Queen, or Proteus Ridley, the SA-X isn't very durable^note, being comparable to the unifinished Ridley robot without the benefit of a guardable weak point. However, its nature as a Mirror Boss with a cheating Screw Attack gives it very high agility, making it harder to hit. It also packs an extremely painful punch, forcing the player to be careful. Its main projectile attack, the Ice Beam, is not easy to avoid without using Screw Attack (which stops you from starting a charge beam, and it can be broken by the SA-X's Screw Attack at the cost of some of its health) and it instantly blows up 2 whole energy tanks. Contact damage with is also very high and it will constantly try to run you down with its own Screw Attack (especially after it notices your own attack patterns) which is as devastating as the Ice Beam. This is further pronounced in its last two desperate forms, neither of which last for long (its monster form goes down in 2 or 3 hits and its core in 6) but they blow up several energy tanks with any of their attacks. Just remember that you can't start a charge with a Screw Attack but can keept a charge with one and try to beat the glass cannon to the trigger.
Heartbeat Soundtrack: Heard when the SA-X is near, but when not in "Chasing you down and trying to kill you violently"-mode. This is basically its Leitmotif.
The Heavy: It initiates the plot of Fusion by escaping containment and is the game's most recurrent and deadly threat.
Humanoid Abomination: It mimics the form of Samus Aran but is a shapeshifting X Parasite. The few glimpses that can be seen of its face show that it's mimicking her human appearance as well, with human-like (albeit blank) eyes, a prominent nose, and (in Metroid Dread) even hair.
Implacable Man
- It relentlessly pursues you once it spots you and nothing short of eluding it or concealing yourself will cause it to break off the pursuit. Even then, there are other encounters, and it is more than likely you will have no choice but to alert it to your presence. You can't harm it in any of these situations. All you can do at that point is run or hide.
- Also in play during the final showdown with it, as the arena given to fight it is actually very, very large, but the SA-X will chase down Samus. Trying to run around is a pretty bad idea too, as the SA-X jumps faster than Samus and it will try to hit you with a Screw Attack at impossible for player angles if you are in the air for too long. Its best to keep fighting it in the flat portion of the area while maneuvering around its attacks as possible, albeit doing a loop around the platforms in the middle can give you one or two second of respite, which is useful if you've dropped low and are panicking.
Irony: The SA-X relentlessly hunts Samus down, just like she once hunted the Metroids of SR-388. Since Samus even has Metroid DNA in her body, it's like she's evading a haunting doppelganger of her past sins. Except this simulacra does not plan on sparing the "last Metroid"...
- Indeed, the SA-X even uses Samus' Metroid battling tactics against her; its standard procedure is to freeze its target with the Ice Beam and then finish with a Super Missile.
Kill It with Ice: Its primary weapon and the reason why it's such a threat to Samus. Averted with itself, even if you can briefly freeze it with Ice Missiles it doesn't actually harm it and it thaws as quickly as a larval Metroid.
Killed Off for Real: Not only does the SA-X end up definitely dead by the end of Fusion with one absorbed by Samus and all of its other copies destroyed with the space station and SR388 being blown to hell, since the SA-X was originally created due to infecting Samus' Power Suit and sampling her DNA, it's actually nearly impossible for the X to recreate that scenario since Samus' integrated Metroid DNA means an X would fail in the best-case scenario, and be absorbed in the more-likely one.
Lightning Bruiser: It's just as maneuverable as Samus, moreso in the air, has all her weapons and has much tougher defenses.
Me's a Crowd: Late in the game, the SA-X is revealed to have split into several duplicates of itself, though you never encounter more than one instance at a time.
Mirror Boss: More so than even Dark Samus, as while Dark Samus will use heavy modifications of Samus weapons, directly weaponize Phazon and various things Samus simply can't obtain on the Prime series, the SA-X's arsenal is literally made up of Samus's stolen weapons. The first portion of the final fight with it is this trope in full effect and is known to be one of the toughest fights in the franchise. Averted once its first form breaks, as it will pull into the classic X bag of tricks by turning into a hulking monster and then finally into its core form.
No Biological Sex: The SA-X isn't female in any capacity even if it's imitating Samus, a human woman (to the point of having her face). It is consistently just called "it". Justified as it is just a parasite using a very powerful form, and the X are creatures that reproduce asexually.
No-Sell: It has Samus's fully upgraded armor on top of adding its own X brand modifications, so nothing can even touch it until the final battle with it. Ice Missiles can stun it for a single second but they don't do any damage. Even during the final battle, you can only hurt it with a fully charged shot of the Plasma Beam. You can't feed it missiles like most other bosses, so while if it doesn't need that many attacks to bring it down, charging and aiming all of them while it rains you down with the hardest hitting attacks in the game is very difficult.
Rocket-Tag Gameplay: It doesn't take much punishment to beat the SA-X in the final encounter ultimately. However, its Ice Beam and Screw Attack return the favor, capable of killing Samus within seconds. And it has multiple forms to make up for Samus having a likely array of extra Energy Tanks in kind. The result is a rush down battle that requires keeping it from getting too rowdy or else you'll hurt real fast.
Screw This, I'm Out of Here!: When Samus destroys its body and shatters its Core-X casing, it flees through the vents rather than be absorbed.
Secret A.I. Moves: Downplayed, but the SA-X can do a Screw Attack without actually having to jump, which is something Samus never could. Notably, it can do it on a straight line, which is deadly during the Final Battle.
Shadow Archetype: To Samus, literally and figuratively. The SA-X is basically if Samus chose to jump off the slippery slope by killing the baby Metroid rather than spare it, and by doing so ensuring the complete extinction of all Metroids and allowing the X Parasites to cultivate.
Shapeshifter Swan Song: It loses its hold on its mimicry of Samus when it takes too much damage, regressing into a bestial abomination with traits taken from her armor and very likely the Hornoad the original parasite was infesting before it ever encountered Samus and finally breaks down into its core form.
Super-Toughness: Unlike Samus herself, the SA-X is completely invincible to beams, missiles, Super Missiles, and Power Bombs. Only the penetrating effect of the Plasma Beam can harm it, and even then only a fully charged shot has any effect. Ice Missiles can freeze it solid, but since it can thaw out faster than any other foe in the franchise, it's not much help.
Villainous Rescue: It flies in after Samus has been reduced to critical condition by the Omega Metroid and fires at it, making it back off and harming it. It's mauled back into its Core-X form in one swipe from the Omega Metroid though. Samus is then able to absorb the X and restore her power. Given the X have been established to be quite capable of critical thought, that it doesn't finish off Samus right then and there and focuses instead on the Omega Metroid, is telling.
The Worf Effect: Throughout the game the SA-X is nigh-unstoppable until the very end, where Samus happens to have all the resources to finally take it down - but not for good. When it appears minutes later to attack the Omega Metroid, one hit subsequently defeats it just like one hit downed Samus to highlight how powerful the Omega is.

Zombie Researchers

Debut: Metroid Fusion

The first results of the X infecting the humans on the B.S.L station. Found on the Main Deck and the entrance to Sector 1.

Blob Monster: A more-or-less humanoid take on this, as they're basically piles of purple goo that shamble about and try to ram into Samus.
Body Horror: Their former human forms are barely discernible through their current gelatinous physiques.
The Goomba: One of the earliest enemies encountered in the game, and by far the weakest; a single shot from your basic Power Beam kills them.
LEGO Genetics: The game manual speculates that the X infected a "slime creature" before infecting humans, explaining the gooey transformations of the victims. Indeed, purple slime creatures are seen throughout the Main Deck next to dead scientists and periodically get re-infected by X to form more Zombie Researchers.
Our Zombies Are Different: They're called such, and they're basically reanimated human corpses.
Shapeshifting Excludes Clothing: They still wear the tattered, ill-fitting lab coats of their hosts. The pants of the victims have disappeared entirely because there's no way those could fit around the glop that replaced the victims' legs.

Elephant Bird

https://static.tvtropes.org/pmwiki/pub/images/mfusionguide_b_01o.jpg

Debut: Metroid Fusion

A Beam Core-X mimicking a Chozo Statue that gives Samus the Charge Beam once defeated.

All There in the Manual: The name "Elephant Bird" is derived from the Prima strategy guidebook.
"Blind Idiot" Translation: The odd name "Elephant Bird" is the result of this. In Japanese, it is simply called 鳥人像 (meaning "bird-person statue", more commonly localized as Chozo Statue), but it seems that translators mistook 像 for the similar kanji 象, which means "elephant" instead of "statue". The guidebook correctly calls it a "Chozo Statue" later on.
Chest Monster: It disguises itself as a Chozo Statue in order to fight Samus. How it mimicked a Chozo Statue is unclear.
Dub Name Change: In Japanese, it's given the same name as all other Chozo Statues. Much like Torizo, the unique "Elephant Bird" moniker is unique to the English guidebook.
Five-Second Foreshadowing: Just before revealing that it's actually a Beam Core-X in disguise, the Chozo Statue's Item Sphere can be shot to reveal the Morph Ball... an upgrade that Samus already acquired at this point of the game. This gives away the fact that the statue is holding a fake item.
Meaningful Name: Elephant Bird shares its name with an enormous extinct flightless bird. Although it's likely unintentional, it fits as a reference to the Chozo being human-sized bird-like species that are nearly extinct and have mostly lost their flight.
One-Winged Angel: Became a Beam Core-X after the orb is shot. Instead of being a Clipped-Wing Angel like other bosses, the Beam Core-X phase is more dangerous than the Chozo Statue phase.
Recurring Element: Elephant Bird follows the series tradition of a Chozo Statue that turns out to be a Chest Monster, following Arachnus in Metroid II: Return of Samus and the Torizo in Super Metroid.
Scratch Damage: If Samus touches Elephant Bird's Item Sphere, it deals 1 damage point.
Warm-Up Boss: As the first Beam Core-X, Elephant Bird serves as an introduction for the subsequent fights with Beam Core-X. It provides an opportunity to teach the player its attack patterns and weaknesses while its Charge Beam mercifully doesn't deal as much damage as later bosses.

Zazabi

Debut: Metroid Fusion

A polyp-like creature mimicked by the X in Fusion. Fought in Sector 2 of the Biologic Space Laboratories and gives Samus her High Jump and Jump Ball abilities.

All There in the Manual: Neither Fusion nor any of its official English guides give this boss a unique name. Its name is instead derived from the Japanese soundtrack.
Go for the Eye: Its single eye is its weak point, but it can't be hit from the outside. Instead, Samus has to shoot through its mouth and guts to damage it.
Goomba Stomp: Its main form of attack.
In a Single Bound: It jumps fairly high for an alien plant-polyp thing. The fact that it holds the High Jump power-up might have something to do with it.
Lamprey Mouth: Located in its underside.
Man-Eating Plant: Despite being clearly based on a Cnidarian polyp, it's apparently a plant. And it tries to eat Samus.
Starfish Aliens: Real-life Cnidarians are already alien in general, and this thing is not any less weird.
Swallowed Whole: This is actually a dumber strategy than spitting Samus out in this case, as its digestive damage is pretty low and its insides are vulnerable to Morph Ball bombs. Still, not getting swallowed feeding it missiles is a better strategy for Samus.
Tactical Suicide Boss: It would be impossible to defeat Zazabi if it didn't try to fall on Samus with its mouth open.

Serris

Debut: Metroid Fusion

"[Sector 4] is home to a very large creature the researchers call Serris. It is capable of moving and attacking at ultra-high speed. The more senseless and widespread destruction here may be attributable to Serris. No doubt the SA-X released it, but I can't be sure why yet. Serris has returned often to the breeding tank here. Its natural behavior must be to go back to it periodically. It's a valuable specimen, but you have been authorized to terminate it. If you don't, it may invade other sectors."

— Adam AI

A giant aquatic serpent. Another creature mimicked by the X. Fought in Sector 4 of the Biologic Space Laboratories and gives Samus the Speed Booster.

Dead All Along: When Samus is dispatched to hunt it down, it's under the assumption that it had escaped and was rampaging through Sector 4. It's not until the last room before its boss fight that she comes across Serris's skeleton, final proof that the X had killed and replaced it.
Dub Name Change: In Japanese, it is named Ishtar. Its name was changed to Serris in the English translation.
More Teeth than the Osmond Family: Serris sports a shark-like set of them.
Sea Monster: As previously noted, giant aquatic serpent.
Super-Speed: Serris becomes super fast when damaged. Incidentally, Samus obtains the Speed Booster after defeating it, which raises the question of whether it's capable of using the ability due to the X that infected Samus or if it's just naturally as fast as Adam implies.
Wake-Up Call Boss: Serris' patterns being hard to define, its only weak point being its forehead, and its Super-Speed when damaged make it a big challenge for unpreparaed players.
Waterfront Boss Battle: Serris lives in a flooded room and attacks by frequently jumping out of the water and moving through the air before submerging again, while Samus stands on one of several platforms above the water or hangs onto an overhead ladder on the ceiling and tries and avoid from there.

Barrier Core-X

https://static.tvtropes.org/pmwiki/pub/images/corex_6.png

Debut: Metroid Fusion

A gigantic Core-X that steals the Varia Suit data from the download station. It's fought in Sector 6.

All There in the Manual: Neither Fusion nor any of its official English guides give this specific Core-X a unique name. It is named in the Japanese soundtrack, which can be literally translated as "Barrier Core-X". Given that "varia" is a mistranslation of "barrier", it could be alternatively translated as "Varia Core-X".
Foreshadowing: It holds the same defensive suit upgrade as the SA-X and the best method for attacking it is the same, or will be once Samus has upgraded enough.
It Can Think: One of the first signs that the X Parasites are capable of strategic thought is that this one not only steals the Varia Suit upgrade, but destroys the station afterwards, attempting to limit Samus's ability to gain power.
King Mook: It's a bigger, more powerful version of the Core-X the other bosses become as their Clipped-Wing Angel form.
No-Sell: It's immune to missiles.
Orbiting Particle Shield: One made of smaller Core-X.
Ramming Always Works: Its only attack, but given its size, capability to pass through walls, and the movement restricting water in the room, it's an effective one.

The Scientist

https://static.tvtropes.org/pmwiki/pub/images/maxresdefault_scientist.jpg

Debut: Metroid Fusion

An X that took over a scientist, planning to kill Samus by detonating the BSL rather than confront her itself. Fought in the Main Boiler Control Room of Sector 3, and carries the Wide Beam.

Bald of Evil: Its disguise as a human was balding and was trying to blow up the entire station to ensure the death of Samus.
It Can Think: The most obvious example of the X, capable of using computers and manipulating the station's systems.
Failed a Spot Check: It knew the best way to set off the stations auto-destruct was a massive enough explosion, i.e. the main boiler meltdown, so disabled its cooling unit. But it's meddling with the system tripped the Emergency Broadcast, alerting Samus and Adam to their plot.
Faking the Dead: Due to its mimicry of the human scientist, though it's unlikely anyone was fooled.
The Needs of the Many: It judged the deaths of all the X on the BSL to be a worthwhile price to ensure the safety of the species as a whole.
No Name Given: Fusion does not name this character, and one of the official English guides merely refers to him as scientist.
O.O.C. Is Serious Business: It's noted that its attempt to blow up the BSL to kill Samus would kill itself, dramatically contradicting the X's normal survival instincts, and showing both how much of a threat they view Samus and that they were no longer mindless creatures.
One-Winged Angel: When Samus confronts it, it sheds its human guise to assume its Core-X form. While this is usually a Clipped-Wing Angel form the X take after a boss fight, since the scientist doesn't actually fight us in its human form, its Core-X form is this trope.
Small Role, Big Impact: Its single brief appearance reveals X absorb its hosts knowledge and memories, and indisputably proves they are capable of higher intelligence. This dramatically raises how dangerous the X are and leads the Federation to take an interest in the X forcing Samus to act against them.
Taking You with Me: It planned to detonate the entire station in a gambit to kill Samus, an act that would have killed itself and all other X on the station, but ensure the survival of its species on SR388.
Was Once a Man: It was once a human before the X infected him. Unlike the Zombie Researchers that appear earlier in the game, it still looks like a normal human before getting shot.

Yakuza

Debut: Metroid Fusion

A giant spider-like creature mimicked by the X. Fought in the Reactor Silo of the Biologic Space Laboratories, and gives the Space Jump upon defeat.

All There in the Manual: Fusion does not name Yakuza in-game. Its name is instead derived from the official Prima strategy guidebooks.
Attack Its Weak Point: Yakuza can only be damaged by shooting its mouth when it opens to attack.
Breath Weapon: It can breathe fire, because being a giant spider thing wasn't already terrifying enough. In its second phase, it starts spitting an unindentified purple projectile (possibly poison).
Didn't Need Those Anyway!: Losing its legs in the first phase doesn't hinder it much, as it can Space Jump around without them, somehow.
Double Jump: It's capable of Space Jumping after losing its legs. Like Zazabi and Serris, it's unknown if this is a natural ability or if it is somehow using the actual power-up due to the X.
Dub Name Change: In Japanese, its name is Gedu. The Prima guidebook changes its name to Yakuza.
Four-Legged Insect: Spiders have four pair of legs, while the Yakuza has three like your typical insect, yet it's still referred as "Spider Boss" in the Prima guide. Justified in that the Yakuza is an alien creature.
Giant Spider: It's a giant arachnid/insectoid creature.
Gratuitous Japanese: Named after the term that describes public Japanese crime syndicate members.
Rugged Scar: On its left eye. It has many eyes, so it's not a big loss for it, but it's still pretty lazy of the X not to fix it.
Spiders Are Scary: With Fusion essentially being a horror game, the intent of Yakuza is to be creepy.
Tactical Suicide Boss: Just like Zazabi, if Yakuza literally kept its mouth shut, it would be impossible to beat.

Nettori

Debut: Metroid Fusion

A strange biomass that is connected to the plant life of the Biologic Space Laboratories. Mimicked by the X as well. Fought in Sector 2 and holds the Plasma Beam.

Alien Kudzu: Plants start infesting Sector 2 and spread to the reactor silo, cutting off all of the space station's power because of it.
All There in the Manual: Fusion does not name Nettori in-game. Its name is instead derived from one of the official English guides while the Metroid Prime & Fusion Original Soundtracks album simply refers to it as a 'Chozo Statue'.
Cores-and-Turrets Boss: Nettori is the core, with pollen-spraying Samus Eaters being its turrets.
Didn't Need Those Anyway!: After enough hits, Nettori loses its arms and then its head.
Green Thumb: It was capable of making the plantlife aboard the research station grow out of control.
Living Statue: It resembles a Golden Torizo for some reason, although some official material refers to it as a "root", suggesting it may some equivalent of a mandrake.
Man-Eating Plant: Its "turrets" are Samus Eaters that slowly digest Samus if she falls in. Not being an insect or tree rodent of any, simply holding down the jump button solves the problem for her.
One-Winged Angel: It becomes a Beam Core-X, allowing it to use its Plasma Beam to attack in addition to ramming while rendering its outer carapace immune to Samus's weapons.
Plant Aliens: If the whole thing about it being a root is true.
Plasma Cannon: It starts attacking with the Plasma Beam in its second phase. The same weapon is the reward for beating it.
Suspiciously Similar Substitute: To Mother Brain. Both enemies are Stationary Bosses whose rooms contain dangerous pools between safe platforms and projectile-firing objects on the ceiling, and defeating both destroys something Samus needed destroyed to complete her mission (the Space Pirate base in Mother Brain's case, the obstructive vines in Nettori's case).
Waterfront Boss Battle: Nettori itself hangs passively above the water, but the water has deadly Samus Eater plants growing in it which attempt to eat you alive and make it difficult to get out of the water if you fall into it.

Nightmare

A hideous amalgamation of flesh and machinery capable of manipulating gravity. Its tropes can be found here.

Security Robot B.O.X.

A security robot on the B.S.L station with an organic processing core. Its tropes can be found here.

Neo-Ridley

An X duplicate formed from the cryogenically preserved corpse of Ridley's clone. For the original's tropes, see here.

ZDR

Chozo Zombies

https://static.tvtropes.org/pmwiki/pub/images/dread_chozozombie.png

When Raven Beak and his Mawkin soldiers returned to ZDR after the massacre of the Thoha, they accidentally brought an X disguised as a soldier with them. The X then consumed every Mawkin on the planet save Raven Beak, who barely managed to contain the Parasites in Elun. Most of the infected Chozo were turned into shambling goo-like creatures.

Blob Monster: Like the Zombie Researchers, they're made from an unspecified gooey substance.
Night of the Living Mooks: Chozo, but as slimy undead things.
Ominous Obsidian Ooze: They take the form of black blobs of goo.
Our Zombies Are Different: Parasitic alien bird zombies.
Suspiciously Similar Substitute:
- To the Zombie Researchers from Fusion. Both are gelatinous X forms derived from sapient species that shuffle back and forth on a single platform. These Chozo are even referred to as "Chozo Zombies" in the game's internal data, strengthening the connection.
- Also to the Chozo Ghosts from Metroid Prime. Both are undead Chozo creatures with slumped-over postures that first appear around the halfway point of the story and start appearing throughout the game world after that first encounter.
Took a Level in Badass: Compared to Fusion anyway, whereas the Zombie Researchers were The Goomba and by far one of the weakest enemies in the entire Metroid series if not for their Resurrective Immortality ability; the Chozo Zombies are substantially more durable and able to do a surprising amount of damage very quickly if allowed to close their proximity. Possibly justified by the Chozo Zombies being made from Mawkin warriors instead of frail human scientists.

Escue

A big insectoid creature that wanders around the Ferenia region, right after the X escape their Elun confinement.

Armored But Frail: Escue has very low health for a boss at the point of the game it's encountered, but half the time it cloaks itself in an electrical field that makes it invulnerable to most attacks.
Big Creepy-Crawlies: Appears to be a stronger X-infected Quetzoa, which resembles a cross between a beetle and a wasp.
Boss Vulnerability: It's usually cloaked in an electrical field that protects it from attacks. However, when it uses its charge move and hits the floor or a wall, it will lose the electrical field and render it vulnerable to damage. It will recharge itself shortly after, however. If Samus has the Screw Attack via Sequence Breaking, she can use it to hurt Escue even when it has its electrical field up.
King Mook: It is a stronger version of the Quetzoa and Quetshocker enemies.
Magic Missile Storm: One of its attacks is firing out a storm of homing electrical projectiles at Samus. Appropriately, defeating it gives Samus the Storm Missiles.
Outside-the-Box Tactic: While the boss room itself falls just short of the distance required to charge a Shinespark, starting a Speed Boost from the previous screen before the fight begins will give Samus more than enough space to activate one, ram into Escue right as it enters the screen, and knock off over half its health, making the rest of the battle a breeze.
Shock and Awe: Like the other Quetshockers in Ferenia, Escue's primary ability is its electrokinesis. Its ramming attack is electrically charged to prevent Samus from countering it, and it can fire a lightning ball that explodes into an electric current if it hits the floor.
Tactical Suicide Boss: If it didn't use a ramming attack that removed its electrical shielding, it would be nearly impervious to most of Samus' arsenal.

Experiment No. Z-57

https://static.tvtropes.org/pmwiki/pub/images/screenshot_20211023_083151_youtube_1.jpg

A giant, reptilian spider-like bioweapon that was developed and experimented by the Mawkin in the Dairon region, before an X managed to infect it. It is fought at Cataris, having messed with the heat generators of ZDR in order to freeze the planet and kill Samus.

Background Boss: Much like Diggernaut from Samus Returns, Experiment No. Z-57 spends much of its battle lurking in the background, and is only vulnerable when its head is in the same plane as Samus.
Breath Weapon: Its main method of ranged attack is to spew a beam of hazardous goop in various methods — via sweeping the floor to cover it in goop temporarily, sweeping a huge section of the room, or to spew it onto a fan which blows the goop towards the opposite wall and also covers it.
Chekhov's Gunman: Experiment No. Z-57 is first seen as a Sinister Silhouette in the background of the Dairon laboratory during the blackout, and then can be seen under full lighting once the power is restored. Although it appears to be deceased, it mysteriously vanishes from the lab after the X are released, setting up its later appearance as a major boss battle.
Combat Tentacles: Its main method of melee attack is to swipe at Samus with its long claw-tipped tentacles, covering huge portions of the area with each swipe.
Disney Villain Death: In its death throes, it falls from the reactor control room into the lava below.
Flower Mouth: Its mouth splits open into three parts like... well, a flower.
Genetic Abomination: The "abomination" part comes from the fact that it looks like a horrifically mutated lizard with a Flower Mouth, spider-like limbs and a slug-like tail, can breath lasers that leave some sort of goop behind, and clogged the heat generator with said goop, despite being much smaller than said generator. The "genetic" part comes from the fact that it was created by presumably the Chozo, with it being made from the DNA of multiple creatures, most obviously a Corpius. This fact is enforced by it not even having an actual name, with it simply being called "Experiment No. Z-57".
Heal Thyself: When it attaches its extra claws onto the walls, it will heal itself at a rapid rate unless Samus quickly shoots down the claws. Fortunately, any claws shot down will not regenerate for the rest of the fight.
It Can Think: If the player hasn't played Fusion, No. Z-57's actions in Dread are the point where it becomes clear the X are frighteningly intelligent. It employs ambush tactics despite its massive size, makes intelligent use of its battlefield in ways none of the previous bosses tried, and is one of the few bosses in Dread (the other being Raven Beak) that Samus actively has to hunt down instead of merely running into it. Its actions actively hamper Samus and were intended to kill her without even a fight due to the X believing at the time Samus was weak to freezing temperatures - which was accurate until only a very short while ago.
Kill It with Ice: The general strategy it had for dealing with Samus, since her Metroid DNA gave her a critical vulnerability to the cold. Unfortunately for the X, Samus just gained the Gravity Suit, rendering the plan ineffective.
A Molten Date with Death: How the Core-X inside No. Z-57 meets its end - succumbing to Cataris's mantle after failing to drag Samus in with it. Although, it's possible that it may have survived and infected Kraid's corpse and eventually Raven Beak as well, since Raven Beak X has a Flower Mouth and extra limbs evocative of Z-57.
Out-Gambitted: It sabotages the planet's central heating to freeze the planet and kill Samus, knowing about her weakness to cold thanks to her Metroid DNA. Unfortunately for it, Samus had recently obtained the Gravity Suit which completely immunizes her from the cold.
Outside-the-Box Tactic: One of its attacks in its second phase has it spew its goop beam on the fan to blow goop projectiles that cover the opposite wall with it, also pushing Samus with the wind and forcing her to constantly run from the wall while avoiding the projectiles. This also gives her an opportunity to charge a Shinespark from all the running, which will One-Hit Kill the boss if she successfully lands it.
Suspiciously Similar Substitute: To Diggernaut from Metroid: Samus Returns. Both are creations of the Chozo who are initially seen dormant in the midst of Chozo ruins early on. They're also both the antepenultimate major bosses of their games who spend much of their fights in the background and have laser and Bullet Hell attacks that require Samus to dodge with the Space Jump.
Taking You with Me: Attempted by Z-57. It tries desperately to grapple Samus as it begins falling towards the lava directly below the thermal vent, but Samus has none of that and ensures it goes down alone.
The Unfought: Not Experiment No. Z-57 itself, but its Core-X, which falls into lava and dies instead of fighting Samus and getting absorbed (or possibly retreats to infect Kraid and later Raven Beak, given the final boss is a mix of Kraid, Raven Beak, and Z-57).
You Are Number 6: Unlike many bosses throughout the series, Z-57 does not have an actual name, which is fitting for a Chozo bioweapon.

Golzuna

A giant mutated Muzby that inhabits the Ghavoran region, where it is fought.

Attack Its Weak Point: Its tiny unarmored head is its weak spot.
Bait-and-Switch Boss: It initially starts as a Muzby-X. Defeating it causes it to mutate into a Goliath. Defeating that causes it to transform into Golzuna, a full-fledged boss.
Big Creepy-Crawlies: It appears to be a stronger Goliath, which resemble giant beetle-like insectoids.
Early-Bird Boss: A rare late game example. The average playthrough will likely pit the player against Golzuna while already having access to the Space Jump, which is helpful against its Cross Bomb attack. However, one can face this boss before obtaining that power up via Sequence Breaking, making it much tougher.
King Mook: To the Muzby and its bigger X-infected cousin the Goliath. The X even takes both forms before adopting Golzuna's final form.
Mighty Glacier: It's huge and damaging, but all its attacks have a long startup time with predictable tells allowing Samus to quickly get out of the way.
Outside-the-Box Tactic: The room where it's fought is big enough to be able to charge a Shinespark if taking advantage of the interval it takes to change forms and use it when it prepares to attack Samus taking away a big amount of health, the same tactic can be used on its Core-X which goes down in one hit.
Shockwave Stomp: Being a King Mook of the Muzbies and Goliaths, its main method of slamming down with its hind legs is a given. It also creates a small shockwave that damages Samus if she is too close.
Take Cover!: Golzuna's huge body blocks the explosions from its own Cross Bomb attacks, allowing Samus to hide in front of it when it uses the non-targeting versions to attack behind itself.
Tiny-Headed Behemoth: Like the ordinary Muzby, Golzuna has a huge lower body and hind legs that are disproportionately large compared to its vulnerable head.
Trick Boss: It initially appears as a regular Muzby, which goes down quickly before turning into a bigger Goliath. When that goes down, it turns into the actual boss Golzuna.

Penultimate X Parasite (spoilers)

Raven Beak X

An X which possesses a weakened Raven Beak and combines his biomass with Kraid, creating a monstrous hulking fusion. For the original's tropes, see here

Final X Parasite (spoilers)

Quiet Robe X

The final X-parasite Samus encounters, possessing Quiet Robe's body. Quiet Robe themself can be found under the Metroid - The Chozo page, these tropes apply specifically to the Parasite:

Heroic Willpower: Quiet Robe X is entirely benevolent, unlike every other living being that's been infected, as demonstrated at the very end where the X copy sacrifices itself in order to return Samus to normal so that she can fly her ship out of the the self-destructing planet. It is unclear whether this is Quiet Robe's willpower making the X-version an ally of Samus instead of an enemy, or if the duplication process ended up replicating Quiet Robe's affection for Samus.
Madness Mantra: Upon assuming Quiet Robe's form, it repeats their last words to her, "I am counting on you", over and over as it reactivates the E.M.M.I.s.
Token Heroic Orc: Quiet Robe's X copy is notably the only non-aggressive X Parasite seen in the entire series thus far, as the copy somehow manages to retain enough of the original's benevolence to sacrifice itself to help Samus in the end. The worst it does is reactivate the E.M.M.I.s shortly after awakening.
Undeathly Pallor: After merging with Quiet Robe's dead body, the X version's skin becomes pale and its eyes glow white instead of orange.

Characters / Metroid - X Parasites

Biological Space Laboratories

ZDR

Raven Beak X

Quiet Robe X

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Biological Space Laboratories

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