The SA-X is an X Parasite mimicking Samus. After Samus's first encounter with the X on SR388, her Power Suit was infected by the parasites. The Galactic Federation surgically removed infected portions of her suit and sent the pieces to the Biologic Space Laboratories research station's Quarantine Bay. Once there, the X inside the infected suit parts seemingly combined and transformed into a copy of Samus in her standard Varia Suit (it should be noted in SA-X's first closeup its eyes appear to be white, with no visible pupils). The SA-X used a Power Bomb to escape the Quarantine Bay, breaching the capsules containing X-infected specimens from SR388, and started the mass infestation.
Powers and Stats
Name: Samus Aran X, SA-X
Origin: Metroid Fusion
Gender: Asexual, but took Samus Aran's appearance
Age: Less than a week old
Classification: Parasite X that absorbed Samus' DNA
Powers and Abilities: Superhuman Physical Characteristics, Enhanced Senses, Scan Visor (Gives out basic information and weaknesses), Precognition, Regeneration (High-Low when suit suffers breaking or cracks), Energy Projection, Weapon Mastery, Vehicular Mastery, Ice Manipulation, Electricity Manipulation, Can absorb and redirect energy projectiles, Can enhance her AP by charging its attacks, Barrier and Armor Bypassing with Wave Beam, Weaponized Plasma, X-Ray Visor, Resistance to Acid, Gravity, and Heat, Speed Boosting, Temporary Invulnerability with Screw Attack, Electro Armor, and Speed Booster, Internal Organ damaging with Plasma Beam, Healing (At least Mid-Low with Crystal Flash and Low with Concentration), Transformation, Possession, Shapeshifting, and Duplication.
Attack Potency: Planet level (Same as fully upgraded Samus in her Varia Suit)
Speed: Unknown (Same as Samus, capable of reacting to Samus' weakest fusion suits)
Lifting Strength: Class 25 to Class 50 (Presumably on par with Samus)
Striking Strength: Mountain Class, likely Planet Class as Monster SA-X.
Durability: Planet level (Easily capable of shrugging off everything Samus threw at it until she was fully upgraded)
Stamina: Virtually limitless
Standard Equipment: Ice/Wide(Spazer)/Wave/Plasma Beam, Charge Beam, Missiles, Super Missiles, Morph Ball, Bombs, Power Bomb, Screw Attack(Exact same weapons Samus had at the end of Super Metroid, since that's most likely what weapons Samus had when she was infected by the X Parasites, since Fusion, Metroid 4, was a sequel to Super Metroid, Metroid 3).
Intelligence: Presumably on par with Samus and Adam claims that it is highly intelligent and is fully capable of wielding any and all of Samus's equipment during the events of Metroid Fusion. However, as an animal it runs purely on instinct and won't do outside of what is necessary to survive.
Weaknesses: Relies purely on instinct, Incredibly weakened by its natural predator, the Metroids.