Changeling

Changelings were individuals controlled by the Graske that mimicked a lifeform and sent the real one to Griffoth. They looked almost exactly like the originals, but their eyes flashed a greeny-gold colour occasionally, hinting at their true disposition. Changelings appeared to behave in just the same way as the originals, so that hardly anybody suspected them. (WC: )

A human mother was once replaced with a Changeling on Christmas Day in the 2000s. The rest of her family seemingly did not suspect the change. She subsequently tricked the father of the family into the kitchen, where a Graske appeared and replaced him with a Changeling too. The real mum and dad were sent to Griffoth, where they were kept alive to sustain their Changeling replacements. On another Christmas Day in 1883, the same Graske appeared in a London street and replaced a urchin boy with a Changeling. A member of the Slitheen family was also replaced with a Changeling at some point. The Tenth Doctor and a human companion arrived on Griffoth and saw all of the individuals these Changelings replaced being stored inside pods. (GAME: Attack of the Graske)

The Pantheon of Discord used Changelings. The Tenth Doctor told the Trickster that he fought his shadows and Changelings. (TV: The Wedding of Sarah Jane Smith)

Behind the scenes

 * The changelings originally appeared in Attack of the Graske.
 * The player could choose from two outcomes with his or her remote control while playing Attack of the Graske. They could return the originals the changelings replaced to their rightful place, or freeze the chamber containing the originals, stopping the Graske but leaving the changelings on Earth.