Could've Been King

The Could've Been King led the Army of Meanwhiles and Neverweres during the Last Great Time War. The Tenth Doctor described this 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)

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

It is unknown what sort of entity the King was.

One possible theory proposed by the Doctor Who community is that the Could've Been King is in fact Omega, the insane co-founder of the Time Lords and their society, somehow free and with an army behind him to take revenge on the Time Lords during the Last Great Time War.