It's a human but they're just pulling the strings behind the scenes for right now.
The reader doesn't need to know who the antagonist is on page one (although it's good to have at least a hint that there IS an antagonist pushing the story along, from the very beginning), but the author does (at least by the time the story is finished, not necessarily in a first draft).
Just make sure you also have someone -- a person -- on the bottom of one set of the strings, opposing the protagonist, so as the protagonist goes through his/her story, he/she is struggling with someone in each scene, on the way to meeting up with the story's antagonist.
Think of it in two interweaving levels: You have a story antagonist, someone who's pulling the strings or pushing the buttons throughout the course of the book and who is defeated (or not, if you're doing an unhappy ending) in the resolution of the book. Plus, each scene has a scene antagonist, who, for a given scene, may be the whole-book antagonist or may be some secondary character. No matter what, though, there are always two people struggling in each scene, and there's a cause-and-effect relationship that links the struggles in each scene, so that the outcome of the struggle in Scene 1 leads to the struggle in Scene 2, which leads to the struggle in Scene 3.... all the way to the final confrontation between protagonist and the antagonist (the one for the book as a whole, I mean).
JD