Arachnus

Arachnus is a species native to SR388. At the near-top of the food chain, they are very fearsome predators, but seem to be very rare as Samus Aran only encounters two of them in the Metroid timeline, one of which is apparently an X mimic. The biped is distinguished by its three-fanged maw, segmented eyes, elongated neck, a hard shell on its back, and the large claws on its forearms. It is known to curl into an armored ball when threatened in order to jump towards its target in an attempt to ram it, and will uncurl periodically to swipe with its claws or spit fireballs from its mouth.

Metroid II: Return of Samus
In Metroid II: Return of Samus, Samus Aran can fight an Arachnus, which is found posing as an Item Sphere in the claws of a Chozo Statue. It is an optional boss, which will give Samus the useful Spring Ball ability upon defeat. Only Morph Ball Bombs will damage it. An easy way for Samus to defeat it is by trapping the beast behind the statue on the left side of the room; she can then enter Morph Ball form and lay bombs every time Arachnus attempts to escape.

Metroid: Samus Returns
In the remake Metroid: Samus Returns, Arachnus' attacks and behavior are greatly expanded to resemble those seen in Fusion, including swiping projectiles, lobbing fireballs and ramming into Samus in ball form in a manner similar to the Boost Ball. It can jump while in ball form, as well as creating a shockwave-producing ground pound. Like in Metroid II, it poses as an Item Sphere in the hands of a Chozo Statue, though its large size and different appearance make its disguise less effective. It will reveal itself if Samus approaches or attacks it, destroying the Chozo Statue in the process. The Arachnus can also be damaged using Beam shots, Missiles and Bombs, the latter of which will cause it the slip, knocking the creature out of ball form. When killed, it will leave the Spring Ball behind once again. It is the first boss Samus encounters that is not a Metroid and one of the three non-Metroid bosses in Samus Returns along with Diggernaut and the final boss.

Metroid Fusion
In Metroid Fusion, the first boss Samus encounters is the imitation of an Arachnus, created when five regular X Parasites and a Core-X merged together. Through gene manipulation, the X have not only changed the creature's pigmentation and increased its size, but have also enhanced its strength and abilities.

Compared to the natural counterpart's simplistic fireball, the Arachnus-X's triggers multiple spirals of fire to continuously emerge from the ground until they reach the other side of the room. The blue projectiles from its claws cover more vertical height that is proportionate to the creature's new size, which now dwarfs over Samus. The Arachnus-X causes a small earthquake whenever it rams against a wall in ball form, causing Samus to lose her grip if she is holding onto a ledge and wound her if she falls directly on top of the organism. Consequently, the imitation has lost much of the quick mobility the uninfected variant on SR388 displayed.

It is a relatively easy fight as Arachnus in Fusion is one of few bosses Samus can damage with her basic, non-upgraded Power Beam, though her newly recovered Missiles are much more effective. If shot with the latter weapon when the imitation is merely walking, it will flail its head back and forth with a distorted visage while roaring in pain. The boss has 150 health, and the Core X has 30. Once Samus defeats Arachnus and absorbs the Core-X out of its shell, she regains her Morph Ball ability.

Metroid II: Return of Samus manual
"It will roll up its body like an armadillo, then rush at Samus."

Metroid Fusion manual
"Native life-form of SR388. They will roll themselves into balls and attack. Dodge and attack with missiles."

Metroid: Samus Returns Official Guide

 * Inhabitants & Eco System of SR388 (p. 23)
 * ARACHNUS
 * "A native life-form that swallowed the Spring Ball Item Sphere and inadvertently absorbed its power, Arachnus is an armadillo-like beast that spins and spits projectiles to attack its enemies. The Spring Ball gives it the ability to jump in the air while curled into its shell. Defeating it shakes the Item Sphere loose, giving you the opportunity to take it for yourself."

One of Arachnus' more common moves is its energy wave attack. Its hands begin to sparkle, then it throws out a wave about the same height as Samus. It often performs this attack up to three times, but it may throw fewer waves. The first wave is easy to dodge, but subsequent waves are trickier. The flashing hands are emphasized on the first wave but largely absent on follow-ups, which can catch even the most self-respecting bounty hunter unaware. The timing between waves is always the same, so focus on getting a feel for it and jumping when your gut tells you the time is right. Arachnus rolls into a ball to perform a variety of attacks. The first is a speedy dash across the chamber. This attack is easy to dodge. Wait for it to spin up for a few seconds, then jump into the air as high as your Power Suit will carry you. You get enough hang time that even a slightly off jump puts you out of harm's way. Like the energy wave attack, Arachnus performs this attack three times in a row before stopping. Until it finishes spinning, you don't have an opportunity to damage it.
 * Walkthrough (p. 66-67)
 * ARACHNUS
 * ''"That wasn't an Item Sphere! That was the armadillo-like native of SR388, Arachnus! Disregarding the Metroids you've fought up to this point, this marks the game's first full-fledged boss battle, and it's quite the challenge. Arachnus can't be damaged if shot from behind or while it's rolled up into a ball; only shots to its red-and-yellow flesh have any hope of harming it. Missiles should be your go-to for this fight, so use them wisely, if you've followed out Walkthrough so far, you should have plenty of missiles to work with. Use your Lightning Armor at the start. It doesn't last more than a few hits, but it gives you a bit of time to get accustomed to Arachnus' attack patterns.

That's not where the ball-based attacks end. Arachnus also loves to roll into a ball and bounce around the chamber. Your best response is to roll up into the Morph Ball and wait for Arachnus to approach you, then roll under it when it jumps into the air. It continues to follow you around the chamber for several jumps, so race to the other side and wait for it to approach, roll under, then repeat. Don't bother trying to wait this attack out by using the Spider Ball to climb walls. Every time Arachnus hits the ground, you're shaken loose and dropped. As with the spinning, Arachnus is invulnerable while bouncing around the chamber.

Its third and final ball attack involves Arachnus jumping into the air, pausing for a moment, then slamming down on the ground. This slam creates a shockwave that damages you if you're caught in it. It's an easy attack to dodge by running away from Arachnus and jumping when it slams to the ground. It only performs this attack once before transitioning into another attack, but you can't damage it until it finishes.

Arachnus occasionally launches fireballs from its mouth in an arcing pattern towards your current location. The ground ignites for a second wherever the fireball lands, so expert to jump away from the fireball, rather than above it. Arachnus shoots three fireballs every time it performs this attack. Dodging these can be tricky, but once you have it down, use the opportunity to shoot some missiles at Arachnus' unarmored regions."''

Trivia

 * The Arachnus encountered in Metroid II and Samus Returns had the ability to jump while curled up in its shell. It may have been using the Spring Ball item that it had acquired beforehand, as the battle takes place near a Chozo Statue and the item emerges from the remains of the creature upon its death. The Metroid: Samus Returns Official Guide confirms this theory. This phenomenon can also be seen in the Ing Sub-Guardians from Metroid Prime 2: Echoes, as they use the power-ups stolen from Samus prior to releasing said items upon their demise.
 * The X Parasite mimic in Metroid Fusion supports this theory - it is never seen leaping into the air in ball form because it does not possess the Spring Ball. If the jumping ability is, however, a natural aspect of the species, the Arachnus-X may not display it in Fusion due to genetic changes caused by the X, as the parasites are known to do with most of their victims.
 * Samus acquires the Morph Ball ability from defeating the Arachnus-X. Based on the fact that the Arachnus species is naturally capable of curling up into a ball, it can be assumed that Samus' recently acquired Metroid genes robbed the natural ability of the creature and implemented it into her Fusion Suit as the Morph Ball item. The power to rob abilities has been displayed by several Metroid strains in the past.
 * As a result, all versions of Arachnus encountered in the Metroid series have rewarded Samus with Morph Ball-related items.
 * The Berserker Lord's theme is an arrangement of the Metroid Fusion Arachnus theme. The Berserker Lord also resembles the Arachnus, and both appear as the first bosses of their games.
 * The Arachnus in Metroid: Samus Returns strangely possesses a new theme. Instead, another remix of the Fusion Arachnus theme plays during the Diggernaut's chase and battle sequence. Because of this, prior to the reveal of the Diggernaut in the Overview Trailer, it was assumed the themes were meant for the battle against Arachnus.
 * In trailers for Fusion, Samus is seen fighting Arachnus in a much larger area than the final game, featuring scenery from Sector 1.
 * Oums resemble the ball form of an Arachnus. The Griptian acts similar to the Arachnus as well.
 * The Arachnus in Metroid Fusion also bears similarities to the Omega Metroid in the same game. Their death sequences both involve them thrusting their heads back repeatedly while roaring and exploding, both are larger forms of bosses found in Metroid II, they both have clawed mouths and claws on their hands, they both have a partly hunched posture and they both walk forwards slowly towards Samus during their battles with her.
 * The Arachnus in Metroid II does not have its own battle music, although this is changed in Samus Returns. It is also the only boss in Metroid II which is not part of the Metroid life cycle. The Arachnus' theme from Samus Returns can be heard here.
 * In the later Japanese version of Fusion (and likely II as well), this creature is known as the "Marukara" (マルカラ) or "Mulcalah", which carried over to the Metroid Prime & Fusion Original Soundtracks.
 * In Metroid Fusion, the Screw Attack can kill Arachnus in one hit, via hacking. This is likely due to its vulnerabilities being the same as a regular enemy, as Samus is able to damage it with the basic Power Beam.
 * The Arachnus' artwork for Metroid: Samus Returns has the exact same pose and "camera angle" as its Metroid II: Return of Samus counterpart.
 * The walkthrough of the Arachnus boss fight in Metroid: Samus Returns Official Guide mistakenly states several times that Arachnus is invulnerable while in its ball form. This is not entirely true as it can be knocked out of its ball form using Bombs. Interestingly, the guide itself never points this out, thus it is likely that the authors where unaware of it. Using bombs against its ball form makes the fight slightly easier as it allows Samus to interrupt its ball form attacks and create an opening for her to attack it.
 * Humorously, knocking Arachnus out of its ball form with a Bomb will cause it to trip and fall in a manner akin to Samus' Slip Bomb special move from Super Smash Bros. for Nintendo 3DS and Wii U.
 * Prior to the release of Samus Returns, the Arachnus-X's claw projectile was believed to be an unnatural addition caused by the X Parasite instead of being an ability present in the Arachnus species.
 * Despite the name, Arachnus does not have any apparent relations with actual arachnids (spiders and scorpions) and instead has more in common with armadillos.

Gallery
Арахнус マルカラ

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