Let's talk antagonists, shall we?
People get confused about antagonists. A lot of writers operate under the misconception that "antagonist" is synonymous with "bad guy" or "evil villain." In many stories, this is true. The antagonist is the dasterdly, scheming, cruel, murderous creep whose primary goal seems to be getting in the way of the protagonist's Quest For Awesome.
Really, an antagonist is just any person (or thing, or force) that opposes the protagonist's goal in the story. If the protagonist wants to save their dad or end a war, the antagonist is whatever is standing in the way of accomplishing that goal. Sure, it can be an evil dictator or an evil step-mother or an evil rabid space monkey, but it can also be more subtle or complex than that. The truly great antagonists of literature are more than just a sweet pencil mustache. Although those things are fun to twirl.
Here are a few alternate antagonists that your protagonist could come up against.
The Foil - This is a character who has opposing qualities to the protagonist, and sometimes an opposing goal. However, they don't have to be super villainous or determined to watch the world burn. A foil can host multiple layers. It's especially effective when the protagonist's own life or personality aspects are mirrored in the foil, albeit in a darker way. Where the protagonist is hot-headed, the foil is cold and analytical. Or the protagonist cares a great deal for their loved ones, and the foil has lost the people the love, which causes them to become vengeful. It's the same trait, and if the reader thinks about it, they might be able to see how even the hero of the story could get to that point if they lost all the people they loved.
The Well-Intentioned - It can be pretty easy to write someone who's just mean or just sociopathic or just empty inside. There's not much to a character like that. They just like pain and/or are disconnected from empathy. That's it. A more difficult antagonist to tackle is the antagonist who honestly believes they're doing the right thing. They don't intend to cause pain, but they feel backed into a corner. And they struggle with it.
The Metaphorical Beast - In some stories, you'll find a creature evolves to become the antagonist. But it's clear this is no ordinary bear or dragon -- this beast represents the bestial part of the protagonist, or something they've lost. The beast poses a definite physical and mental threat, but there's even more at stake. By slaying their white whale, what will the protagonist overcome? Will they destroy themselves in the process?
Mother Nature - Sometimes the antagonist is Nature itself. A storm tears the town apart. A series of earthquakes threaten the protagonist's family. A blizzard will kill them unless they find shelter and sustenance. Like the metaphorical beast, inclement weather often plays into a larger theme within the story. Weather alone can be pretty boring if left to its own devices, so this antagonist has to be spun just the right way to be effective.
Mankind - Like Mother Nature, Man can be an antagonist. Or rather, the things mankind creates. All the stories where the main obstacle is war, or cannibalistic corporations, or the slow death of humanity because the environment is dying... sometimes we create the bad guy ourselves.
The Supernatural - This is the catch-all category for all your zombie apocalypses and faceless demon armies. Usually these sorts of ambiguous supernatural creatures are substantiated with one of the other antagonists on this list, because by themselves, they're not all that interesting. Unless you spin them the right way, of course. Some authors select one primary focal point to represent what they're up against -- the super-fast zombie, the weretiger with the limp.
The Protagonist - Yes, the protagonist can be their own worst enemy. This choice is often used in connection with another external factor, but eventually the protagonist realizes they weren't struggling against something else this whole time, not really. They were always struggling against themselves. This can be something as simple as learning to believe in themselves, or as complex as having to literally face themselves at the final showdown. Maybe it's about overcoming grief, or facing a piece of themselves that they're not ready to face. Maybe they're becoming the thing they're supposed to be fighting against.
Remember, stories can have more than one antagonist. Many do. Do you have a favorite antagonist? Why? Tell us about them in comments!