The Ancient One

First appearance: Well, that's a bit tricky...
When Marvel first started publishing its Official Handbook of the Marvel Universe, the Ancient One's first appearance was listed as being in "Strange Tales #111".

(this entry from issue #13, cover-dated February 1984),
but that ignored this from Dr. Strange's initial story in Strange Tales #110 (July 1963):




NOTE: The pages from Strange Tales #111 can be seen in the thread for Baron Mordo.

However, somewhere along the line, it appears that the Ancient One was retconned into being the "sick lama" who gave Doctor Druid (originally "Doctor Droom") his mystical abilities:


from Amazing Adventures #1 (June 1961),
as was recently reaffirmed in 2019's History of the Marvel Universe issue #2:


NOTE: Weird Wonder Tales #19 was the issue that reprinted the Doctor Droom story from Amazing Adventures #1 and reworked it so that the character would be Doctor Druid.