Merlin (Dr. Who's Time Tales)

One account depicted Merlin as an immortal magic-user. He spoke with a Germanic accent and had a prominent nose. In the Middle Ages, he wore a long black robe embroidered with stars, and a pointed cap to match. In the 20th century, he had cut his beard short and wore less conspicuous clothing, including a fez. During the former period, he had a pet raven.

His "black magic" included a spell that could transport a man through time. Surviving into the 20th century, he had to go into hiding, living as the obscure, elderly owner of a curiosity shop in the Balkans. He once accidentally left his centuries-old, highly magical diary in a public area of the shop during open hours; when an American man tried to buy it, and opened it despite Merlin's warnings not to, he found himself transported back in time to the Middle Ages, briefly exchanging words with the past version of Merlin. Recognising him as a man from the future, sent back in time by Merlin's own future self, the medieval Merlin sent him back to his home era as the Laws demanded. Back in the "present", the man found that his hair had become a snowy white, proof positive that he had not simply dreamed the whole thing. Merlin resolved to be more careful about leaving clues to his identity around in the future. (COMIC: Dr. Who's Time Tales)

Behind the scenes
The Creation of Camelot showed that in the Doctor Who universe, the title of Merlin was passed on from one individual to the next, being more of an office in King Arthur's court than a name. Individuals known to have held the title of Merlin included an otherwise-unknown human being who died "on the job" and an incarnation of the Master (both in The Creation of Camelot), the Eighth Doctor as well another, otherwise-unnamed renegade Time Lord (both in One Fateful Knight, with the Eighth Doctor also reusing the alias in The Shadows of Avalon), and most strikingly, a mysterious future incarnation of the Doctor mentioned throughout Battlefield, potentially the last-ever incarnation of the Doctor.

Thus, it is unclear if the "Merlin the Wise" seen in Dr. Who's Time Tales was one of these various established Merlins, or another claimant to the name altogether — although he, of course, does not resemble the Eighth Doctor or the "Merlin" Master, crossing off two suspects. Although it could not have been the authorial intent, as Dr. Who's Time Tales predates Battlefield, there is actually a wealth of coincidental evidence pointing towards to Merlin's possible identity as a future Doctor: he has knowledge of time travel and the Laws of Time, keeps a large leather-bound diary (much as the Doctor did), and wears a fez.