Polymorph (Red Dwarf)

A polymorph is a shape-shifting organism that can change into anything it pleases, and lives by draining a person of a negative emotion. It was designed to be the perfect warrior, blending into any background, causing confusion and disarray amongst the enemy by playing with their emotions, but the creature mutated into something terrifying its creators couldn't control. One of the polymorph's forms appears somewhat similar to the Xenomorph alien from the movie Alien.