Those pigs are pretty antagonizing, but especially Snowball and Napoleon. They start off as proles under human control but quickly ooze into leadership after the Rebellion. Then the antagonism beings: stealing food, abusing the animals, and ultimately becoming indistinguishable from the oppressive humans they once fought to overthrow.
In general, power corrupts the pigs; they don't start off as antagonists (think Old Major—not a dislikable dude!), but rather become antagonists as the story develops. Well, maybe not Napoleon. We think he might have been bad from the beginning.