The Terrible Tinkerer
The Terrible Tinkerer (an early Spider-Man Villain) is a perfect example of how a villain (or any other character) can be retconned into something very different from the original intent of the writer who created him. In this case, Stan Lee, the Tinkerer’s creator, used a common trope (aliens hidden among us), to introduce Spider-Man’s first (and, at the time, only) alien foe. As first presented, the Tinkerer was an alien disguised as a human repairman who was spearheading an alien invasion. He only had one Silver Age appearance, in Amazing Spider-Man #2. However, he was later retconned to be, in fact, a real human who had tricked Spider-Man and others into believing he was an alien in order to stay hidden and unknown. As the retcon goes, the Tinkerer is actually the tech guy/armorer for many supervillains.
As retcons go, this one was actually worthwhile, and helps explain how so many bad guys have their hands on such good technology.
1. Amazing Spider-Man #2 -vs. Spider-Man (Note: 1st Appearance is in first of two stories in this issue)
2. Amazing Spider-Man #160-(1976-After Silver Age)
While Tinkerer only had one appearance in the Silver Age, he was retconned later to have been human (in ASM #2, he was portrayed by Lee and Ditko as an alien disguised as a human), who had disguised himself as an alien to escape detection. The retcon, however, also turned him into a significant part of the underground supervillain society in the Marvel Universe. As it turned out, Tinkerer was the “tech guy” for many supervillains, including Mysterio (he made Mysterio’s suit), Scorpion (made Scorp’s tail), among many others. Other villains the retconned Tinkerer has worked with/for include Hammerhead, Jack O’Lantern, Black Cat, the Beetle, the Owl, the Jester, Diamondback, Whirlwind, Grizzly, and the Latverian government of Countess Luciana Von Bardas, among many others.
Sources:
https://www.spiderfan.org/characters/tinkerer.html
https://en.wikipedia.org/wiki/Tinkerer