Shakri

 were supposed myths in Time Lord history, but became true when the Eleventh Doctor met a hologram, referred to by the Doctor as a "propaganda poster", aboard a ship hovering over the Earth who had deposited mysterious cubes onto the surface in an attempt to bring the human race to extinction before they could colonise space.

Biology
to be added

History
The Doctor explained that the Time Lords were taught about the Shakri when they were young, but were believed to be legends. They existed at all moments in time and space. (TV:  The Power of Three)