Baron Heinrich Zemo (villain)

First appearance: an uncredited (at the time) role in a flashback in
The Avengers #4 (March 1964):



After that, he was seen in The Avengers #6 (July 1964):






while at the same time, he was appearing in Sgt. Fury (and His Howling Commandos) #8 (July 1964) in a story taking place during WWII and referred to as "Dr. Zemo".



And from The Official Handbook of the Marvel Universe #13 (February 1984),