Thunderbolts

Fair warning, while this is about Marvel's original Thunderbolts team...

...there are some spoilers below for those who might not have read the comics.

After Onslaught, the Fantastic Four and the Avengers had disappeared off the Earth.  

Luckily, these new heroes, the Thunderbolts arrived...but why did Citizen V, Mach-1, Songbird, Techno, Atlas and Meteorite seem vaguely familiar?

Because...

...as revealed in Thunderbolts #1 (April, 1997) by Kurt Busiek and Mark Bagley...

...extra spoiler space...

...they were really the Masters of Evil!  

Baron Helmut Zemo (son of the original Baron Zemo) hatched the plan to pretend to be heroes and took the identity of Citizen V, with the Beetle (Abner Jenkins) as Mach-1, Screaming Mimi (Mimi Schwartz)  as Songbird, the Fixer (Norbert Ebersol) as Techno, Goliath (Erik Josten) as Atlas and Moonstone (Karla Sofen) as Meteorite!

As the series by Kurt Busiek and Mark Bagley went on, some of the villains stayed villains, as others came to enjoy their heroic identities.

Below, are the debut issues of the original members of the team....

Baron Zemo (as Phoenix, then Baron Zemo) Captain America #168 and #275

.

Moonstone (as Karla Sofen, then Moonstone) Captain America #192 and Incredible Hulk #228 

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Goliath (too many identities) and Screaming Mimi (Iron Man Annual #7 and Marvel Two-In-One #54)

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Beetle and the Fixer (Strange Tales #123 and #141)

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Thunderbolts for a while, but also Masters of Evil!