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Metroids are a genetic experiment made by the Chozo because the Chozo needed a creature to feed on the X parasite (as suggested in the Manga and Metroid Fusion) but however government groups and terrorist groups found a unique aspect to these creatures. The unique aspect is that the metroids can suck the life force out of creatures and they can improve themselves so that they can survive better on the planet they are on (as suggested in an unused scan in Metroid Prime 3)