War Doctor

This version of the Doctor appeared at the end of The Name of the Doctor. It is unknown which incarnation of the Doctor he is.

This regeneration of the renegade Time Lord was initially seen within the time line of the Doctor. The eleventh incarnation of the series protagonist spurned the very memory of this new incarnation, going so far as to suggest this incarnation did not even hold the moniker of "the Doctor" due to his actions. The unknown incarnation himself defended those actions by stating they were in the name of peace.

Speculation as to the specific generation of this time lord suggests that he is actually the true 9th incarnation of the Doctor and the actions being referenced were the events leading up to the end of The Last Great Time War.

Regardless, the inclusion of this unknown regeneration into the canon of the Doctor Who mythology would at least make the known regenerations of the Time Lord one further regeneration along (Christopher Ecclestons Doctor being the 10th incarnation etc).