Merlin (One Fateful Knight)

Among others, the name of "Merlin" was used by a Renegade Time Lord distinct from the Doctor.

He had a round face with a "puckish" smile, and wore flowing white robes. He knew the First Doctor before he left Gallifrey. After running away in a TARDIS which even the Doctor claimed was an "old piece of junk", he set out to cause as much damage to the structure of history as possible, believing that the order pursued by the orthodox Time Lords was inimical to life.

Travelling to Camelot in England's past, he used an extension of his TARDIS's chameleon circuit to allow Uther Pendragon to take the form of Igerna's lawful husband Gorlois (allowing Uther to father Arthur), presenting himself to Uther as the wizard Merlin. He then began giving the knights of Camelot Sontaran energy weapons and other such advanced technology. However, the Eighth Doctor arrived in Camelot on the day of the baby Arthur's birth and took over the "Merlin" alias. The two Time Lords had a confrontation which ended with Merlin fleeing in his TARDIS.

Merlin reappeared several decades in Camelot's future, around the time of a grown Arthur's war with Mordred. Capturing the Eighth Doctor, who was still posing as Merlin, the Renegade used his chameleon circuit tricks to take on the Eighth Doctor's own appearance and resume his activities as Merlin, seeking to frame the Doctor-as-Merlin for Arthur's murder. However, he was intercepted by Morgaine, of whom the Doctor, in his "Merlin" guise, had made an enemy; Morgaine trapped Merlin, alive and conscious but in suspended animation, in a column of ice in the middle of Arctic Desert, there to remain until the polar ice caps of Earth melted to free him. (PROSE: One Fateful Knight)