8.

Alien, monster, horror from outer space... call it what you will, the Thing was a being from imagination of John Carpenter. Locked away in the deep-freeze of the Antarctic ice, this parasitic organism came to life when thawed. On a cellular level, the Thing could mimic the cells of it host, producing a near identical copy as means of deception. When cornered, it would turn into its true form, which was typically a grotesque amalgam of other creatures it have been. Is it so inconceivable that a mimic like this might evolve on another planet? Its reproductive success would surely be high.