The retroactive name given to the Doctor’s incarnation between the Eighth and Ninth Doctors is the War Doctor. Played by John Hurt, this incarnation fought in the Last Great Time War between the Time Lords and the Daleks.
The War Doctor was created for the 2013 50th-anniversary story "The Day of the Doctor." His existence was revealed at the end of "The Name of the Doctor," and his origin was shown in the mini-episode "The Night of the Doctor," in which the Eighth Doctor chooses to regenerate into a warrior.
He initially rejects the name “Doctor” because of the violence he believes the war requires. The anniversary special later revisits his decision to use the Moment, a superweapon capable of destroying Gallifrey and the Daleks. The story places him directly between Paul McGann’s Eighth Doctor and Christopher Eccleston’s Ninth Doctor, making “War Doctor” the canonical quiz answer rather than an informal fan label.