Mecha Samus is a character in Metroid Avenger 3: Vengeance. Originally a scrapped co-op feature of the early Metroid 1.5 document, it has been reworked into Vengeance.
Mecha Samus is just that, a mechanical Samus. At a certain point of the events of Vengeance, there would be a sidequest where Samus Aran and Roy Dane search for components to create an android double of the former. She gives it commands and it fights alongside her. It is non-playable.
In the final battle with Ferris Wren, Mecha Samus is damaged enough that she is barely able to move on her own free will, and so she stumbles into a hydraulic press which Wren crushes her in, but not before she could put an explosive on Diana Dane's cage to destroy it.