The Doctor (The Brain of Morbius)

An incarnation of the Doctor was seen during the Fourth Doctor's mindbending contest with Morbius. (TV: The Brain of Morbius, AUDIO: Cold Fusion) According to one account this man was an incarnation of the Other, whom the First Doctor was a reincarnation of. (PROSE: Lungbarrow)

Behind the scenes
The face of this Doctor were first introduced in TV: The Brain of Morbius but it was not until AUDIO: Cold Fusion forty years later, in 2016 that they were confirmed to be incarnations of the Doctor.

However, the incarnation of the Doctor generally considered the first described himself as "the original, you might say" (TV: The Five Doctors), implying that he was the first incarnation of the Doctor. Additionally, at the end of his life, the Eleventh Doctor detailed how he had used all thirteen of his regenerations, including all eleven "main" incarnations, the War Doctor, and the Tenth Doctor's aborted regneration that created the Meta-Crisis Doctor, with no mention of an additional incarnation beyond the limits of a standard regeneration cycle (TV: The Time of the Doctor).