I'm throwing in the unexpected enemy. The Best Friend. The Faithful Sidekick. The person on whom the protagonist most relies. The person who agrees with the ends but has equally strong convictions that thwart success during the means. Perhaps the BFF doesn't believe in the quest at all, rather they believe in the relationship...until the quest threatens that relationship. Perhaps the BFF antagonist is trying to save the protagonist from self-destruction. A Best Friend antagonist could be the character embodying the protagonist's path not taken, the belief not held, the ability to bend, etc. Whatever the contrasting characteristic, I often classify these antagonists as:
The divergent hero.
That kind of antagonist can be a tricky character to write and still have the reader respect--if not like, without liking too much. After all, why, after the second or third time of BFF cockups, does the protagonist still hang around? On the other hand, how do you keep the BFF from overtaking the story and becoming the true protagonist? Maybe you're doing it deliberately to set up your plot twist? In which case the protagonist is, in fact, the antagonist.
Some of our stronger Real Life relationships are with people who are similar to us in many ways, yet we all know how one of those differences can change the entire relationship--sometimes permanently. It's common and even necessary to have minor conflicts between the protagonist and his Scooby-gang; but pushing the conflict with the BFF into the realms of plot-twisting stakes-raising, full on clash of characters...well, that's where an antagonist is born. The antagonist doesn't have to be evil, they just have to be different enough to have a conflict of method or means.
There you have it, readers, the antagonist who isn't a villain, but is instead the confidant, the ally, and the best friend. Who is your favorite fictional frenemy?