According to Robert Kirkman, Image Comics almost passed on publishing their biggest hit in, uh, pretty much forever. Yes, at first Image didn’t want to do The Walking Dead, because in their eyes it was just another zombie story. In black and white no less! Who’s going to buy that?
So how was Image finally convinced? With aliens of course. Kirkman told Image it would eventually be revealed that the zombies were the work of aliens looking to weaken earth’s defences before invading. Kirkman says he never actually planned to use this storyline, or at least that’s what he says now — the comic ended up being pretty much an immediate success, so he never had to answer any questions from Image higher-ups wondering where those aliens they were promised were at.
Also, in the same interview Kirkman revealed he doesn’t have a specific ending for The Walking Dead worked out yet — so, you know, those aliens could still be on the table.