Shakri

 were supposed myths in Time Lord history, but became true when the Eleventh Doctor met a hologram, referred to the Doctor as a "propaganda poster", aboard a ship hovering over the Earth who had deposited mysterious cubes onto the surface.

Biology
to be added

History
The Doctor explained that the Time Lords were taught about the Shakri when they were young. However, they were believed to be legends. (TV:  The Power of Three)