Sarkovian

The Sarkovians were a species that the Doctor was familiar with.

When the Eleventh Doctor examined a Krovian Memory Trap, he speculated that it may have been Sarkovian technology. (COMIC: Fooled)

The Eleventh Doctor, Amy Pond, and Rory Williams once fought a Sarkovian warlord. (PROSE: Paradox Lost)

The Twelfth Doctor once told Clara Oswald that he had "faced oblivion at the hands of the Sarkovian War Splinter". (COMIC: Selfie)

The Twelfth Doctor later told Hattie Munroe that he could show her a flotilla of Sarkovian Nightships. (COMIC: Playing House)

Right before visiting Grant and his family in New York, the Twelfth Doctor stopped a Sarkovian invasion in England. (COMIC: Ghost Stories)

Behind the scenes
, the Sarkovians have only been referenced in stories written by George Mann.

Mann's short story The Moments In Between in the charity anthology Seasons of War lists "the rites of a Sarkovian death march being read on a constant loop" as one moment of history ripped from the fabric of time by the War Doctor's TARDIS to be preserved from the devastation of the Last Great Time War.