The X are parasitic organisms discovered on SR388 that can infect any living creature. They kill their hosts and absorb their DNA to assume the host's form. An X that has possessed multiple hosts can combine their appearances and traits during the process of replication and morph into a new entity.

While created through different means, both are shown to be powerful adversaries who tested Samus' abilities. However Dark Samus was shown to have more personality, while SA-X was simply an X Parasite mimicking Samus at her full power.

Quiet Robe X appears before her, and despite Samus immediately raising her guard by pointing her arm cannon at him, he greets her in a similar manner as he did in life, turns into a red X and willfully lets Samus absorb him to neutralize her powers to allow her to touch the controls without draining it, and she managed ...

Metroid Fusion[edit] At the start of Metroid Fusion, whilst Samus is on a mission by the Galactic Federation to escort researchers upon the planet SR388, she encounters an X-infected Hornoad. She easily destroys it, though the leftover X-parasite flies into Samus.