These gelatinous, gravity-defying parasites originated from planet SR388. Due to their impeccable mimicry capabilities, combined with a potential to multiply infinitely in an extremely short period of time, the X are not only a threat to the ecosystem of the planet, but the entire galaxy. The Chozo recognized this and created Metroids in order to quell their threat on the planet. After Samus destroyed the Metroids on SR388, the X-parasite no longer had a natural predator, and were able to proliferate once again.
After the advent of the Metroid vaccine and Samus's new Fusion Suit, the X-parasite becomes Samus's source of replenishment. Its most basic form comes in four different types: Yellow, Green, Red, and super-cold Blue.