Shape thief

"Shape thief" was an imprecise name the Doctor, Steven and Dodo once applied to a metamorph they encountered in Napoleonic Russia. The Doctor was never able to positively identify the species of the individual.

Indeed, if its description of its history was accurate, it may have been mutated into a metamorph, rather than being from a species of natural metamorphs. In fact, the creature had a natural loathing for scientists because of how often they poked, prodded and tested him in his early life.

Its usual modus operandi was to change into the body of someone of significance — or, as Steven pointed out, a "winner". Then it could amass power and influence for itself.

At one point during the encounter in Russia, the shape thief changed into the Doctor himself, in an attempt to steal the TARDIS. Despite being an excellent copy of the Doctor, however, the TARDIS' isomorphic controls detected a difference between the metamorph and the real Doctor, thus preventing the being from commandeering the TARDIS.

The creature was ultimately defeated, because the Doctor planted a seed of untruth. He deceived the shape thief into believing that Napoléon Bonaparte's assault on Moscow was a success. Tempted by the notion of mimicking the future ruler of the world, he changed into Bonaparte. Unfortunately, by this point, the real Bonaparte had fled Moscow for his long retreat to Paris. The creature, however, made the mistake of running out into the night, whereupon he was quickly captured by angry Muscovites, who believed they'd captured the real Bonaparte. He apparently died, seemingly of his own choice, at their hands. (CC: Mother Russia)