Most Supreme Captain Trenkor Lep was a Sloathe.
When the freighter it was on was damaged, it was killed but its remains still were animated. After the wreckage of the ship crashed on Planet X, what was left of Lep grew and merged with the planet, growing ever hungrier. To stop it, 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!)