The antagonists and protagonists switch throughout the novel. In the beginning, Old Major and then Napoleon and Snowball lead the group of protagonists: the animals themselves. From this perspective, Mr. Jones is the main and initial antagonist. He is their owner and oppressor. They are essentially his slaves. Old Major sets the stage to rebel against Mr. Jones. In Chapter 2, Jones gets drunk and forgets or neglects to feed the animals. They help themselves to the food. Jones and his workers attack and the animal rebellion has begun. Jones is chased off the farm and the animals gain control. Jones and the other farmers continue to be antagonists to the animals. However, within Animal Farm itself, a new division of antagonists and protagonists emerges.
The pigs begin to gain too much control over the other animals. (This is an allegory of the Russian Revolution. Old Major represents Lenin, Snowball represents Trotsky, and Napoleon represents Stalin. The Russian Revolution started out with promise and the support of the people. But it soon became corrupted with too much power at the top.) With the growing power of the pigs, the other animals become more poorly treated. This increases exponentially when Napoleon ousts Snowball from the farm. By the end of the novel, Napoleon has become a dictator. He has become as bad, if not worse, than the original antagonists: the humans. With Napoleon running the farm with an iron fist (or hoof), he then becomes the animals' primary antagonist. He makes them work harder than they ever have and he gives them less and less to eat.
At the end of the novel, there is a striking scene showing how Napoleon and the pigs (once protagonists and now antagonists) have now become identical to the humans:
The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.