Could've Been King

The Could've Been King led the Army of Meanwhiles and Neverweres during the Last Great Time War. He was described by the Doctor as one of the potential horrors that would be released when the Master broke the time-lock on the war, along with the Nightmare Child and the Horde of Travesties. (DW: The End of Time)

One interpretation is that The Army of Meanwhiles and Neverweres under the command of the Could've Been King are metaphors for the armies fighting in the time war where the Could've Been King is whoever destroyed the rest. For example Davros could have been the "Could've Been King" with his dalek army that neverwere but meanwhile are fighting in the time lock or Rassilon and his time lord army.

Beyond this mention, there is no further description of the Army of Meanwhiles and Neverweres or the Could've Been King.