Not-thing

The Not-Things were entities from beyond the universe that imitated the Fourteenth Doctor and Donna Noble when the TARDIS deposited them on a spaceship at the edge of creation.

Being capable of replicating individuals almost completely, including their memories, the Not-Things were capable of copying Donna up to a 99.9% accuracy level. The Doctor claimed they accomplished this by converting the surrounding heat into mass, making the surrounding air cold when within their presence. However, as they acclimatised, they could do so without noticeable heat change. They copied best at speed, working off of fear and the adrenaline response. They had trouble with shaping human arms and jaws specifically.

Being entities from outside the universe, they initially did not understand size, shape, object permanence, or the intricacies of the human psyche. This made them easier to identify, however this became more difficult as they learnt more about their target as they continued to replicate them. By the end of their encounter with the Doctor and Donna, one was able to copy Donna to 99.9 percent, but her wrist was 0.06 millimeters off which the TARDIS could detect. Both entities were killed when the ship self-destructed. (TV: )