Doctor Who has seen its titular Time Lord defend the Earth from all manner of alien invaders. While some, such as the Daleks and the Cybermen, have returned time and again to menace the human race, some other enemies of the Doctor have only appeared for a single episode. While some monsters seen on the sci-fi series may only warrant a one-off appearance, others certainly have potential to return for a rematch.
Source：https://www.cbr.com/doctor-who-racnoss-return/
