The Metarex, or as they are called in Japan, Metalex (メタレックス Metarekkusu?), are an army of cyborgs and robots in the television series Sonic X. They are led by the villain, known as Dark Oak and have one motive: to control the galaxy (or so it seems). They are similar to the Black Arms and Dark Oak is similar to Black Doom from Shadow the Hedgehog. The regular soldiers look similar to the common Black Arms soldiers. The Metarex serve as the main antagonists in the final season of Sonic X, almost replacing Doctor Eggman, although unlike Eggman, the Metarex had everything laid out in advance so that they could both protect vital operations and attack enemy worlds at the same time. Most of the important Metarex take their names from different trees. They are also known to be very dangerous and vicious as they'll do everything just to destroy all planets in the galaxy even if it means losing part of their army.
The Metarex are a race of robotic marauders, equipped with highly advanced and powerful weaponry. They have traversed the galaxy, stealing the so-called "Planet Eggs" and objects of great power, leaving whole worlds dead or dying in their wake. Among their victims are Cosmo the Seedrian's home-world, along with every member of her species except her. They also implanted Cosmo with a hidden sensor in her brain, turning her into an unwilling spy.
The Metarex have a plan known as Operation Duplicate. A large part of this plan involves creating Chaos Emerald duplicates, with the power of the originals. These copies, however, seem to phase out after use. This plan also involves using Chaos Energy to mutate Planet Eggs, growing them to unusual sizes. Their plans also include wiping out non-plant life in the universe, leaving plants to rule. The leaders of the Metarex are, in truth, male Seedrians, which means that they are of the same species as Cosmo. Their robotic-like shells were created by using Planet Egg energy, from their own planet, for the result of a crystallized evolution. Dark Oak and his commanders are the only Metarex who are not entirely mechanical; while their soldiers are automatons, the commanders are simply encased in thick armor.
Ultimately, the Metarex goal is the galaxy's Forestation—the eradication of all life except that of plants. To achieve this, the Metarex need the Planet Eggs and Chaos Emeralds to supply water and light. So nourished, they require the power of an event that occurs once every thousand years in the center of the galaxy. With the energy, the Metarex would be able to wipe out all non-plant life.
Some Metarex are very powerful, too strong for Sonic the Hedgehog and his allies to defeat without assistance from either an ally, some sort of device, or a Power Ring. There are multiple types, and they appear in every environment. Many Metarex wield a blade weapon of some type, and most come equipped with missiles as well.
- Dark Oak (leader)
- Red Pine
- Black Narcissus
- Pale Bay Leaf
- Yellow Zelkova
- Dr. Eggman ("Honorary Metarex" for a short period of time, mostly as a double agent in order to find information about Metarex so that it can be used against them later)
In addition to Dark Oak, the Metarex commanders and Scarship, numerous other Metarex were featured in the course of Sonic X. Most of these are nameless, but a number have designations.
- Metarex Troopers - Humanoid Metarex with insectoid heads who serve as the standard troopers of the Metarex. They are capable of flight, but are otherwise unremarkable.
- Metarex Carrer - The Metarex Carrer was first Metarex besides Dark Oak to appear, this Metarex had a mostly triangular appearance. Appearance: single dark blue eye, with a matching orb on the chest. Equipped with pincer - ended tentacles and electrical blasts. Later showed up to escort Monkey Metarex, but was irreparably damaged by the Blue Typhoon.
- Rocket Launcher/Weapon attacker Metarex - A boxy Metarex dispatched by Dark Oak to destroy the Blue Typhoon. Had an array of missile/rocket launchers built into his chest as well as a yellow sword and beam cannons on the ends of his fingers. He was also capable of folding into a flight mode when in the air. Destroyed by the first use of the Sonic Driver.
- Fish Metarex - Amphibious Metarex troops resembling brown piranha with arms and legs.
- Big Fish Metarex - A larger, blue version of the Fish Metarex who served as their commander. It was armed with a trident and torpedoes. Destroyed through use of the Sonic Driver.
- Frog Metarex - Metarex in the shape of a gigantic frog with a razor - tipped tongue and retractable spikes on its chest. Destroyed the Egg Mars, but was defeated by the Blue Typhoon.
- Ice Metarex - Metarex with a body made of inorganic ice. Capable of extending limbs to form razor - sharp ice spikes as well as having freezing breath. It grew more powerful through the use of an artificial Chaos Emerald.
- Monkey Metarex - A rather unimpressive, cowardly Metarex in the form of a large monkey. Dispatched by Dark Oak to retrieve a Planet Egg, which resulted in the planet becoming a jungle. Robbed of his bodyguard, the Monkey Metarex attempted to defeat Sonic and co. with a series of traps. He self-destructed once everyone teamed up against him.
- Nanite Metarex - One of the more powerful Metarex; composed of an indefinite mass of nanites. Using it's ability to alter its form, it created a haunted castle and nearly succeeded in defeating Sonic and co. It was also capable of harnessing destructive energy. Its core was destroyed by Shadow the Hedgehog.
- Naga Metarex - Numerous Metarex with humanoid torsos and snake - like Tails. Utilized in the operation of an artificial Chaos Emerald manufacturing plant. Defeated by the combined efforts of the crew of the Blue Typhoon. Have a resemblance to the Hydralisks of the game Starcraft.
- Firefly Metarex - Tiny glowing firefly like Metarex used to locate Chaos Emeralds in episode 62.
- Dragon Metarex - Giant, dragon - like Metarex with spiked balls on the end of their tails and rocket launchers on their lower torsos.
- Dinosaur Metarex - These kinds of Metarex patrol the first Metarex base that Sonic and friends come across in Sonic X Season 3. There are two kinds; giant, blue tyrannosaurus rex-like Metarex with clamper-like feet and mouths, and black pteranodon-like Metarex.
- Silver Metarex - Prototype Metarex Trooper created by Black Narcissus to test Sonic's speed; destroyed by Dark Sonic. Similar to Bocoe.
- Gold Metarex - Prototype Metarex Trooper created by Black Narcissus to test Sonic's strength; destroyed by Dark Sonic. Similar to Decoe.
- Dark Oak is voiced by Matt Hoverman in English and Jōji Nakata (as Dark Oak) and Katsuyuki Konishi (as Lucas) in Japanese.
- Red Pine is voiced by Jonathan Todd Ross in English.
- Yellow Zelkova is voiced by Ted Lewis in English and Takeshi Watabe in Japanese.
- Black Narcissus is voiced by Sean Schemmel (who also is the voice of Black Doom) in English and Ken Narita in Japanese.
- Pale Bay Leaf is voiced by Jim Napolitano in English.
- The word "Meta-" is a prefix that means "change or transformation," or "one level or description higher," and the word "Rex" is a word that means "royal" or "king".
- The Metarex leaders are named after plants or trees. They would eventually be revealed as the male versions of the Seedrians.
- Due to their appearances, their designs may have been based on the robots from the long-running series, Transformers.