x parasite
A gelatinous and nigh-unkillable parasite introduced in Metroid Fusion, native to the planet SR-388. Capable of asexual reproduction and perfect mimicry after infecting and killing a host, utilizing all memories prior to the host's demise. There is no known limit to how many forms can be consumed and utilized by a single X, resulting in enhanced abilities should it desire more strength.
They would have stood at the top of the universe's food chain, if not for their bio-engineered predator- the Metroid.