The SA-X is a secondary character of the Metroid Fusion title of the Metroid series. It is a variation of the X Parasite that absorbed the title character, Samus Aran's DNA and abilities, and used them to release itself from captivity, founding the basis of the game.
The SA-X from Metroid Fusion is believed to have come from the Quarantine Bay on the Main Deck. Because it absorbed Samus' abilities, it used a Power Bomb to breach the protective capsule it was in and the capsules containing other X Parasites. With the Ice Beam and Samus' other abilities, it easily opened the Level 0 hatch guarding the bay and left to go to one of the sectors. This can be evidently seen when entering the Quarantine Bay area where there is a warped wall. The bay is the first mission of Samus'.
There are a number of SA-X encounters and discoveries in the game. Shortly after recovering the Morph Ball from a standard core-X, as Samus is going down the personnel elevator to head to Sector 1 (SRX), the SA-X blows a hole in the left wall, walks out, stares at the camera, where a close up of its face is obtained, then walks over and missiles the hatch, destroying it. Samus then feels a shudder as the hatch explosion occurs. The computer then confirms that an unknown agent is at work. The first sighting of it is in Sector 2 (TRO), where it confirms the computer's statement that the thing is equipped with the Ice Beam.
Why Samus Couldn't Destroy the SA-XEdit
The reasons why Samus could not destroy the SA-X is because of the fact that the SA-X has her fully-powered suit, equipped with the Ice Beam and such, would easily be able to destroy her at any point in the game. Due to her increased vulnerability due to the Fusion Suit, she cannot face it head on. Her computerized CO notes that however weak she is, a penetrating weapon such as the Plasma Beam may be able to destroy it.