Not-thing
From Tardis Wiki, the free Doctor Who reference
The Not-Things were creatures from beyond the universe that imitated The Doctor and Donna Noble when the TARDIS deposited them on the Spaceship (Wild Blue Yonder). Being capable of copying individuals completely, including their memories, the Not-Things were capable of copying Donna up to a 99.9% accurate level. They copied best at speed, working off of fear and the adrenaline response. They had trouble with shaping human arms and jaws specifically. (TV: Wild Blue Yonder)
