In the various Sonic the Hedgehog continuities, there are several instances where the characters encounter extraterrestrial life.
Seedrians (Cosmos in the Japanese version) 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 round, 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. However, Earthia invented a crystal which prevents going to final phase.
For untold years, the Seedrians were locked in a war against an unknown 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, including Daisy's lover, the planet's leader Earthia, devastated at what they had become, decreed that they would have to leave planet Green Gate 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 Earthia, creating a colony upon which Earthia'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, the Metarex 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 Earthia's daughter Cosmo. After the defeat of the Metarex, which involved the casualty of all known Seedrian survivors, the Seedrians are virtually extinct.
- The Seedrians bear a great resemblance to the Pokémon Ralts and its evolved forms, Kirlia, Gallade and Gardevoir. 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 aforementioned Pokémon.
- The Seedrians also bear a great resemblance to the creatures from "Maginaryworld", 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.
- Because they are plant-like creatures, it is possible that the Seedrians can be reborn. In the English dub, Cosmo lets Tails know of this fact before he had to sacrifice her.
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; Yellow Zelcova and Red Pine. 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.
The Marmolimians are aliens that hail from the Planet Marmolim. They resemble short creatures with heads that are either round, shaped like a Chao's head or stretched. They also have small tufts of hair on their heads. The Marmolimians are apparently religious, frequently using magic-based charms in love and fortune-telling. Their only appearance was in the 69th episode of Sonic X, where they helped the relationship between Tails and Cosmo somewhat. Marmolimians possess ships that resemble meteors. They also use a coin-based currency called Fleebos. Four Marmolimians named Lu, Leela, Momo and Gana (the latter is the spiritual elder) formed the crew of the ship that they owned.
Sonic the Hedgehog comics