Alien Species

In the various Sonic the Hedgehog continuities, there are several instances where the characters encounter extraterrestrial life.

Black Arms
''Main article: Black Arms.

The Black Arms, essentially the darkest and most powerful of the aliens mentioned, are the only aliens to appear in the game continuity. They appeared in the game Shadow the Hedgehog as the primary enemies or allies, depending on the player's path. Their leader is called Black Doom, who is a Black Arm that floats, wears a brown coat and has celestial charms all over him. When they first appear, there is no explanation, the player soon learns that they came for the Chaos Emeralds. The Black Arms appear to have their own technology which revolves around guns (that can range from crude to seemingly expertly crafted) that emit purple, fatal laser beams (with the exception of one, which shoots worms.) The Black Arms (or, rather, Black Doom, as he's the only one that can talk) claim that they have come to save humanity from itself, but their true intentions are more parisitic in nature.

The Black Arms also appear in the Archie-published Sonic the Hedgehog comics, where they entered into a war with the Xorda. Their role is essentially the same as it is in the game, and Shadow was created to protect Mobius from them.

Seedrians
Seedrians is a fan term for a species of plant-like aliens who have evolved to assume sentience and more animalistic forms that appear in the Sonic X series. Male Seedrians, referred to by the fans as Thorndrigans, are slightly larger than most of the inhabitants of Sonic's world, and are usually rather rotund, with short tails, small antler-like growths on their heads, and a colored ridge on the top of their heads; they are largely similar to trees in their make-up. Female Seedrians, or Petali, more closely resemble their plant ancestors, being mostly slender and having an arrangement of petals sprouting from their waists and buds--or bloomed flowers--atop their heads. Apparently Seedrians evolve into trees at some point in their lifetimes, perhaps a first stage of death or old age.

For untold years, the Seedrians were locked in a war against a mysterious foe. In an effort to combat them, all the known males joined forces under the command of a renegade scientist named Lucas and willed themselves to undergo unnatural evolution, becoming bestial giant versions of their former selves. However, the evolution process was unstable, and in many cases fatal. Stealing their own world's Planet Egg, they used its power to stabilize the evolution process. Finally, after many died, the planet's leader Hertia, devastated at what they had become, decreed that they would have to leave the planet to survive and abandoned her world and her mutated comrades to destruction by their enemies. Since this enemy is never revealed, it is a constant focus of fan fiction. The most common belief is that it was the Black Arms mentioned above.

However, this caused dissension, as the male Seedrians, now known by the term Beasts, were unwilling to abandon the fight. They continued to do battle until only five Beasts, including Lucas, remained alive, those who were to become Dark Oak and his henchmen through a finishing phase of their evolution and became the leaders of the Metarex. A mass number of the females however, had escaped into space with Hertia, creating a colony upon which Hertia's last daughter, and possibly the last child of the Seedrians, Cosmo, was born. Determined to rid the universe of all non-plant life, and to have revenge on their former colleagues, they hunted down and attacked the ship carrying the surviving females, all but exterminating them.

The ship was destroyed, leaving only six known specimens of the Seedrian race alive: Lucas, reborn as Metarex leader Dark Oak; his commanders Red Pine, Yellow Zelkova, Black Narcissus, and Pale Bay Leaf; and Hertia's daughter Cosmo.

Trivia

 * The Seedrians bear a great resemblance to the Pokémon Ralts, Gardevoir, and especially Kirlia. In fact, certain Pokémon fans who are not familiar with the Third Season of Sonic X tend to confuse the female Seedrians as "probably fan-altered" forms of the mentioned Pokémon.


 * The Seedrians also bare a great resemblance to the creatures from "Maginary World", a dreamworld in Sonic Shuffle. For example, Void highly resembles Cosmo. This could simply be a coincidence or an attempt to bring back Void's people to the show, and it is even possible that the two are somehow related, although this is highly unlikely.

Metarex
Main article: Metarex

The Metarex are an alien race of bio-robots. The five most powerful and high ranking Metarex, Dark Oak and his commanders, are actually the only suriving male Seedrians. Desiring the power to unlock transformations that would enable them to defeat their enemies, they stole energy from their home world's Planet Egg and were transformed. From that point on, the Metarex sought to achieve domination by destroying all animal life in the universe, allowing plants to rule over all.

Bem
A scientist race from the Archie-published comics, the Bem all appear to be identical females, possibly due to cloning. The Bem travel the galaxy, and on several occasions their journeys have brought them to Mobius. The Bem recovered Shadow the Hedgehog from falling into the atmosphere, restoring him to health and giving him the energy to achieve Super Shadow to hold off the Xorda while they escaped. Later on, Sonic visited the Bem planet of Argentium, also known as Wheel World, where he found Amadeus and Rosemary Prower. Sonic eventually used a Bem wormhole to return to Mobius, finally ending the banishment imposed on him by the explosion of the Quantum Dial. Under the impression that Sonic was a renegade Bem, but unwilling to pursue his one-seater ship into Xorda space, the Bem allowed him to escape. Some time later, Sonic returned to Wheel World with Knuckles and Tails to find it in ruins with the Xorda and the Black Arms fighting in orbit.

The most prominent of the Bem is Ceneca 9009, the first scientist to develop a De-Roboticizer, also referred to as the Robotizer.

Xorda

 * See main article for more details.

A highly aggressive alien race who attacked Mobius when it was still known as Earth. As a result of their bombardment, ordinary animals were transformed into Mobians. Upon learning that the planet still had life, they returned and attempted to destroy the planet. Thanks to Sonic and an alliance of every fighting inhabitant of the planet, their efforts were thwarted.

While they might have become aware of Mobius' survival, the Xorda have since become locked in battle with the Black Arms.

The Drakon
Main article: Drakon Empire

A race of warlike aliens appearing in the UK publication Sonic the Comic, the Drakon are a race of technologically advanced fish-like aliens that evolved on the extra-dimensional planet Drak and used their scientific prowess to design and build armoured suits that provided mobility. In ancient times, they waged a war with the ancient Echidna race led by Pachacamac for possession of the Chaos Emeralds, but were eventually defeated by unknown methods and driven away. They presumably spent the next eight millenia conquering their own dimension before they encountered Dr. Robotnik, who convinced them that Sonic was a criminal who had to be brought to trial. This would lead to several clashes between Sonic and the Drakon Empire, who were eventually double-crossed by Robotnik and sent back to their own universe.