Ouroboros

The Ouroboros was the symbol of a snake or worm (PROSE: Falls the Shadow) eating its own tail, representing all of time, i.e. eternity. This was an Ouroboros loop, according to Bernice Summerfield. (AUDIO: The Tears of Isis)

Demesne Furze, the Great White Worm/Wyrm, was worshipped by the Romans and served as the origin of the symbol on Earth. (AUDIO: Trail of the White Worm)

The signature emblem of the Corsair was an Ouroboros. According to the Eleventh Doctor, he had the image tattooed on his body after each regeneration, even when he regenerated as a female. Without it, the Corsair didn't feel like him/herself. (TV: The Doctor's Wife) Jan Rydd had an ouroboros tattooed around his neck. (PROSE: Love and War)

The Mara was described as ouroboros, (PROSE: Antony and Cleopatra) as was the Timewyrm which was said to circle the universe and bite its own tail. (PROSE: Timewyrm: Revelation)

The Oroborus entity (TV: Oroborus) and the Oroborous Worm (PROSE: The Mary-Sue Extrusion) were named after the symbol.