Zombies in general.
Here here. When I started writing a book 2.5 years ago, it had zombies in it (sounds like a great book, eh?). Then the zombie zeitgeist took over, and I went back and completely excised all of it. Replaced them with cannibals because the cannibal fad is not upon us yet (Sounds even better now, right?).
And to think...this is a book about mountain climbing.