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.

The Quiet Robe copy then reassumes its red X form and deliberately lets itself be absorbed by Samus, giving her access to Thoha genes and regaining full control of her Metroid powers so she does not endanger her Gunship.

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.

Each game ultimately ends with the Metroids going extinct, with them always returning too. Until Metroid Dread, where the Metroids truly are well and gone...