Sarkovian

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

Sarkovian sculptors were believed to have created The Newborn, a statue of the Human-Dalek hybrid Dalek Sec exposed in the Space Museum. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

The Eleventh Doctor, Amy Pond, and Rory Williams once sought Archibald Angelchrist's help in gathering materials to help them fight a Sarkovian warlord. (PROSE: Paradox Lost) When the Doctor examined a Krovian Memory Trap, he speculated that it may have been Sarkovian technology. (COMIC: Fooled)

Sarkovian bilejuice was served at Joe's Diner. (COMIC: Midnight Feast)

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