PolyMorph
Created to be the perfect killing machine, the Polymorph can assume any shape it wants, to push its victims to the very edge of whatever base emotion it requires sustenance of, and then allow it to suck out that emotion. When one comes aboard Red Dwarf, it first attacks Lister, pushing him to the brink of his fear and then leeching out that emotion from him, leaving Dave literally fearless (and also brainless, for a man without even just a little fear is truly stupid!). It then goes on to take the Cat's vanity, leaving him a derelict with no regard for himself, Kryten's guilt, allowing him to supercede his inbuilt obedience to humans, and finally Rimmer's anger, leaving him a peacenik!
The very nature of the Polymorph makes it difficult to locate, as it could quite literally be anything, or anyone. When the boys finally dispose of the creature they believe they are safe, but some time later they come across another version of the Polymorph, an Emohawk, and again they must do battle to save themselves from becoming hors d'oeuvres on the creature's menu of emotional cuisine!
