The Doctor (The Brain of Morbius)

Eight incarnations of the Doctor from before the First Doctor were seen during the Fourth Doctor's mindbending contest with Morbius. (TV: The Brain of Morbius, The Timeless Children; PROSE: Doctor Who and the Brain of Morbius, Cold Fusion; AUDIO: Cold Fusion)

Legacy
When describing his "renewal" process to Ben and Polly, the newly-regenerated Second Doctor implied that he might have previously renewed himself in the past, and he opened a trunk that contained relics from his previous incarnations: Saladin's ornamental dagger; a large earring he used to wear; a solid gold-like thick bracelet with odd pictures; and Cameca's jade brooch. (PROSE: The Power of the Daleks)

When Polly, Ben, and Jamie found a shaving mirror on the control console of the second control room in the Doctor's TARDIS, Polly and Ben speculated that the room may have been used by a bearded incarnation of the Doctor from a time before either of the Doctors they were familiar with. (PROSE: Something at the Door)

During the mindbending battle, after Morbius said, "Back! Back to your beginning!" and the eight faces began to flash on the screen, the Fourth Doctor thought to himself, "You can't... not that far... I won't let you... Not even I." (PROSE: Cold Fusion) The mechanism broke down before any more faces could be shown. (TV: The Brain of Morbius)

The Fifth Doctor only had hazy memories of his life from before what he remembered to be his second regeneration; further back, great chunks were missing. (PROSE: Cold Fusion) The Sixth Doctor also mentioned that his memory was vague prior to his regeneration into the Third Doctor. (COMIC: The World Shapers)

The Eighth Doctor saw a series of men in pseudo-Edwardian clothes in the Tomorrow Window. (PROSE: The Tomorrow Windows) After being trapped in the Matrix by, the Thirteenth Doctor escaped by overloading the system with memories of her past incarnations and of all she had seen; these eight faces, as they had appeared to the Fourth Doctor during the mindbending contest, were among them. (TV: The Timeless Children)

Appearance
In order they appeared on the mindbending device following the First Doctor; it is worth noting that the screen had previously counted down form the Fourth Doctor to the First. (TV: The Brain of Morbius)

First incarnation
He was fair-skinned with a dark moustache and dark straight hair of mid length with a top hat. (TV: The Brain of Morbius)

Second incarnation
He was fair-skinned with no facial hair but curly light-coloured hair of mid length held at the back with a bow tie. He wore a tricorn hat and had a light coloured necktie with a top with dark collar. (TV: The Brain of Morbius)

Third incarnation
He was fair-skinned with light coloured curly hair and a full beard. He had a dark top with no collar at the front but a very high stand behind the head. (TV: The Brain of Morbius)

Fourth incarnation
He was fair-skinned with light coloured curly hair and a dark full beard. He wore a light coloured necktie with a top with dark high collar. (TV: The Brain of Morbius)

Fifth incarnation
He was fair-skinned and beardless but had long curly dark hair. He wore a dark cavalier hat with a light coloured feather. (TV: The Brain of Morbius)

Sixth incarnation
He was fair-skinned, had a goatee and moustache and straight dark lengthy hair. He wore a dark wide-brimmed hat and a top with light coloured collar. (TV: The Brain of Morbius)

Seventh incarnation
He was fair-skinned and beardless but had lengthy dark straight hair. He wore a dark wide-brimmed hat. (TV: The Brain of Morbius)

Eighth incarnation
He was fair-skinned and had a short dark full beard which went lighter at his temples. He wore a dark narrow brimmed hat with a dark feather. (TV: The Brain of Morbius)

Behind the scenes

 * The faces included (in order of "earliest" to "latest" incarnation): Christopher Barry (director), Robert Banks Stewart (writer), Christopher Baker (production assistant), Philip Hinchcliffe (producer), Douglas Camfield (director), Graeme Harper (production assistant), Robert Holmes (script editor), and George Gallaccio (production unit manager). (DWM 329, 541) Although they are sometimes wrongly attributed.
 * The montage of The Timeless Children uses a different order: the six of the seven Timeless Child incarnations of the episode from earliest to latest skipping the sixth one, then Philip Hinchcliffe, Christopher Baker, Robert Banks Stewart, George Gallaccio, Robert Holmes, Graeme Harper and Douglas Camfield's incarnations, the elderly Brendan as played by an uncredited actor, and finally Jo Martin's incarnation.
 * Many other stories would suggest that William Hartnell's Doctor was, in fact, the earliest. (TV: The Three Doctors, Mawdryn Undead, The Five Doctors, The Time of the Doctor, et al.) The novel Lungbarrow proposed a possible solution for this contradiction: following persecution from Rassilon, a Time Lord founder threw himself into a Loom to be resurrected, and the Doctor had many of his memories; this would suggest that the faces in the mindbending contest belonged to this founder rather than the Doctor himself, though this was only hinted and not explicitly stated in the novel as its foreword is Morbius' quote from the mindbending contest. Similarly, The Timeless Children, taken in its entirety, implies (though it does not state) that these eight Doctors were some of the lives the Doctor experienced as regenerations following their Timeless Child self, and prior to being regressed into a child and having their memories erased, both of these events being established in the same story.
 * The original rehearsal script by David Whitaker for TV: The Power of the Daleks (then known as The Destiny of Doctor Who) stated that the Doctor renewed before, and that last time, he was wearing a metal bracelet stored in a drawer of the console, also containing a large earring he used to wear at some point. (DWMSE 4) The earring and bracelet were described in John Peel's novelisation of the serial.
 * Philip Hinchcliffe, producer of TV: The Brain of Morbius and himself one of the faces, said on the faces: "We tried to get famous actors for the faces of the Doctor. But because no one would volunteer, we had to use backroom boys. And it is true to say that I attempted to imply that William Hartnell was not the first Doctor." They all wore stock costumes. (REF: In-Vision #12: The Brain of Morbius, A History of the Universe) In Classic Who: The Hinchcliffe Years as well as A Day with Philip Hinchcliffe, Hinchcliffe reconfirmed this original intention and, while noting that many fans tried to say that the faces were Morbius' rather than the Doctor's (a claim repeated in Doctor Who The Handbook: The Fourth Doctor), he was certain that he had played the Doctor.
 * Script-editor and uncredited co-writer Robert Holmes, also one of the faces, confirmed Hincliffe's account: "We don't know which one Hartnell was, whether he was the first or not. In the phantasmagoric scene where they are mind-wrestling, we see the Doctor forced back through a number of regenerations." (REF: In-Vision #12: The Brain of Morbius) He would later introduce the idea of a twelve regeneration limit in TV: The Deadly Assassin, a story which also established that the Doctor's contemporary the Master had reached his last life as well, although, of course, it is possible for the Master to have burned through regenerations at an accelerated rate, the interpretation that mainstream continuity eventually settled on.
 * Issue 12 of In-Vision also included photographs of seven of the eight "Young Doctors".
 * Lance Parkin, author of the novel Cold Fusion, confirmed that Patience's newly-regenerated husband was intended to specifically be the Douglas Camfield Doctor (REF: AHistory); in an earlier draft of the novel The Infinity Doctors, Patience's husband would have been the Robert Banks Stewart Doctor who regenerated into a Geoffrey Bayldon incarnation at the end.
 * A screenshot of these Doctors' appearance in the mindbending contest appears in Strax Saves the Day when Strax notes that The Day of the Doctor featured "all the original Doctors, except for most of them".