Okay, actually the second Man-Thing. The character debuted in a generic, black and white monster story in Savage Tales #1.
It was beautifully illustrated, but it’s not part of official Marvel canon. In fact, the story had written many years earlier as a one-off by Stan Lee and Roy Thomas. Lee has taken credit for the name (and it may be one of the best character names in history, in truth), while he and Roy created multiple possible origin stories.
But ultimately, the character didn’t become part of the Marvel Universe until Thomas took his done-in-one story and integrated it into Astonishing Tales #12-13, along with Ka-Zar and A.I.M.
Len Wein and Neal Adams do a little flashback in the middle of the story as well, in back and white and yellow.
Creators: Roy Thomas and John Buscema, with a flashback sequence by Len Wein and Neal Adams