The X parasite clone of Samus in Metroid Fusion is called the SA-X. It forms from an X parasite that copies Samus and the parts of her Power Suit removed during emergency surgery.
The X can absorb a host’s genetic information, appearance, memories, and abilities. Because the infected suit retains Samus’s equipment, the SA-X resembles Samus at full strength, including powerful weapons and upgrades that the weakened Samus has lost. This makes it one of the game’s most dangerous threats.
The SA-X is not Dark Samus. Dark Samus is a separate entity associated with Phazon and the Metroid Prime games, while the SA-X is an X-based imitation from Metroid Fusion. The name is commonly understood as shorthand for “Samus-X,” reflecting both Samus’s identity and the parasite responsible for the copy. The game later reveals that the SA-X can reproduce, meaning more than one copy exists aboard the B.S.L. station.