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The amorphous creature is a common trope in science fiction. Usually it is depicted as a living mass of jelly, slime or liquid-like substance that can take any shape it wants.
Amorphous creatures are to be distinguished from shape-shifters that can change their appearance due to their body material to mimic a living or non-living thing however they like.
Metaphorically, "amorphous" refers to any structure, body, figure, text or speech that lacks a distinct form or order.
Aliens from the Sector General novel series by James White are among the most diverse. Crystalline methane creatures, continent sized carpets, rolling ring-shaped aliens, chlorine breathers, radiation eating telepaths and creatures assembled from several symbiont species meet in a setting of space hospital.
"Blob" (The Blob) Perhaps the most familiar example of an amorphous extraterrestrial creature
C'tan - 'Star Vampires' worshiped and given corporeal form by the Necrontyr, only four still exist, only two of which (the Nightbringer and the Deceiver) are active, Mars (The Void Dragon) is believed to be one that was covered by space dust over trillions of years ago. (Warhammer 40,000)
Q - all individuals of the Q Continuum which is accessible to the "Q" and their guests, and the true nature of it is said to be beyond the comprehension of "lesser beings" such as humans so it is shown to humans only in ways they can understand. (Star Trek)
Watchers In The Dark (highly telepathic creatures that inhabit the ruins of Caliban, home of the Dark Angels Space Marine Chapter) (Warhammer 40,000)
Human Is is a science fiction short story by Philip K. Dick in which a woman whose cold and emotionally abusive husband returns from a survey mission to the dying planet Rexor IV, and is changed for the better - it is later revealed that his psyche was replaced by a Rexorian, glad to have escaped the confines of its dying planet.
In Invasion of the Pod PeoplePod People grow in large seed pods and gradually take the form of a particular person, eventually taking over their bodies once the growth process is complete
The 1953 short story Colony by Philip K. Dick features a predatory alien life form that is capable of precise mimicry of all kinds of objects. The size and complexity of the mimicked object can varies with the larger ones usually trying to trap and consume humans similar to carnivorous plants.
In the 1954 short story The Father-thing by Philip K. Dick aliens replace and mimic humans. Dick's story is typically more personal because it is not about the (alien) invasion of a community, but of a family.
Slitheen (or, more accurately, Raxicoricofallopatorians — Slitheen is a family name, not the species), bipedal, vaguely humanoid creatures with sharp claws and baby-like faces, made of living calcium (Doctor Who)