There were some pretty wild regenerations in the classic series for 'Doctor Who' - the
Second Doctor having regeneration forced upon him by the Time Lords; the Fourth Doctor being inhabited by the Watcher. Even after the process settled down to be a uniform explosion of fire and rebirth with the regeneration of Eccleston's Doctor into Tennant's, the mythos has backdated itself to finally show the regeneration of the Eighth Doctor into the War Doctor with a new variation.
Sir John Hurt played the War Doctor in an online mini-episode "Night Of The Doctor". He didn't film anything new for this production; in fact, most of the seven minute episode dealt with the last moments of life for the Eighth Doctor.
Hurt showed up only as an image in a reflection, when he was much younger man. It is assumed that it came from a 1979 TV adaptation of "Crime And Punishment" and it's striking to see the age difference with that of John Hurt as seen in the 50th Anniversary Special "The Day Of The Doctor".
Hurt has noticeably aged over 30 years - in human chronology. But who knows how many decades he was in that body as he fought his one-man war against both the Time Lords and the Daleks in the attempt to get them to cease their hostilities against each other?
After Cass' spaceship crashed on the planet of Karn, the Sisterhood of Karn found the Eighth Doctor's body and brought him back to life for four minutes, long enough to make the decision about what type of Time Lord he should choose for his next regeneration. And they had prepared for that moment with a selection of elixirs that would trigger and lock in the next incarnation of the Doctor.
Having seen how hatred of the Time Lords led Cass to face death rather than be rescued by one, the Doctor was shaken to his core and he no longer wished to be known as the Doctor. Instead he wanted to choose a regeneration that would make him a warrior.
And Ohila, the High Priestess of the Sisterhood, had the perfect potion for him, one that she had concocted herself. Drinking it, the Eighth Doctor regenerated into the War Doctor, whose first words were "Doctor no more......"
We don't know what goes into the Sisterhood's elixir to trigger so specific a regeneration for a Time Lord, but I believe I know where she got the ingredients for each of the offered elixirs, to be fat or thin, young or old, even a choice in gender.
I believe the Sisterhood raided the DNA date banks in the Matrix of Gallifrey.
More than that, I think her special concoction featured the DNA and personality attributes of one specific Time Lord.
I think that younger version of John Hurt was a manifestation of how the Master looked in one of his earlier incarnations.
We never saw the Master until he was already in his last incarnation, the Thirteenth, as played by the late great Roger Delgado. Which means there are twelve incarnations of his life still out there in the Time-Stream whom we never met.
The Master certainly would have held the attributes needed for a Warrior throughout all of his incarnations, probably the one constant in his personality after that of his megalomaniacal desire to rule the Universe. And the High Priestess would have had the skills to winnow out the quirks of his personality that would have defeated the purpose for her planned offering of a new regeneration.
So that's my story and I'm sticking to it. The War Doctor was the physical manifestation of an earlier incarnation of the Master.
As to why the Doctors played by David Tennant and Matt Smith didn't recognize the face of "He Who Must Not Be Acknowledged"? Perhaps it was an incarnation of his old enemy who never crossed the path of the Doctor. Or better yet, by the time they met him, the War Doctor was so old and weathered that he no longer resembled his younger self.
I can see John Hurt when I look at his 79 year old face; but I can't see the man I remember from 'I, Claudius', "Alien", or even "History Of The World, Part One". How many centuries separated the Doctor's memories of the past incarnation of the Master from how his borrowed face looked on the War Doctor? I wouldn't be surprised if he couldn't place the face.......