Six was a Sloathe.
On an incursion to Rubri, Six was wounded and abandoned. It was nurtured to health by the native polymorphs, whose form it eventually adopted. It eventually became a crewmember of the Schirron Dream. Six sacrificed itself by attacking the TARDIS with its phobia-inducing sting. This caused the TARDIS to lash out and kill Six and destroy the thing that had been Trenkor Lep, saving the TARDIS and its occupants. (PROSE: Sky Pirates!)