Skeletor was a name which Donna Noble used to refer to a Sycorax (COMIC: The Widow's Curse), Meng Tian (COMIC: The Immortal Emperor) and Death. (AUDIO: Death and the Queen)
The Doctor uses it as a possible alias of The Master when talking to Rossiter and Addams: "Tell me quickly, what's going on? The Master, Harold Saxon, Skeletor, whatever you're calling him, what's he doing up there?" (TV: The End of Time)