Ouroboros

The Ouroboros was a symbol of a snake eating its own tail.

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)

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