Those pigs are pretty antagonizing, but especially Snowball and Napoleon. They begin as animals, members of the working class under the control of the humans, but quickly step up to a position of leadership after the Rebellion. They just get more antagonist-like after that: stealing food, abusing the animals, and ultimately becoming indistinguishable from the oppressive humans they once fought to overthrow. The pigs are corrupted by the power they seize; they do not begin as antagonists (think old Major – not a dislikable dude!), but rather become antagonists as the story develops.