You may be looking for his main universe counterpart.

Arthur was King of Camelot in the 8th century in a parallel universe.

Together with his knights and Merlin, Arthur battled the forces of Mordred and Morgaine, but was killed at the Battle of Camlann. Merlin, believed to be a future incarnation of the Doctor, placed Arthur's body and Excalibur in a dimension-crossing spaceship and hid it at the bottom of Lake Vortigern, near Carbury, in rural England. Legends sprang up that Arthur had survived the battle, albeit badly wounded, and would reawaken at the time of England's greatest need. (TV: Battlefield)

The Eighth Doctor witnessed the birth of legendary king Arthur Pendragon. Remembering his encounter with Morgaine in his seventh life and the destiny that he must fulfil, the Doctor began calling himself "Merlin" and defeated Morgaine and a renegade Time Lord, who wanted to overpower Camelot. However, he was too late to stop the renegade from killing Arthur. The Doctor used Morgaine's spaceship to place Arthur's body at the bottom of the lake, leaving the message for his seventh self, ensuring that both his and Arthur's destinies would be fulfilled. (PROSE: One Fateful Knight)

In 1997, the Seventh Doctor and Ace discovered Arthur's spaceship and retrieved Excalibur. They later discovered that stories of Arthur's survival were just propaganda and that the King really was dead after all. (TV: Battlefield)