Upon looking at the final paragraph of the book, you should see that it is a tragic ending:
"Twelve voices were shouting in anger, and they were all alike. No question, now, what had happened to the faces of the pigs. 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."
The animals on Animal Farm, who had been overworked and mistreated by the humans, had once started a Rebellion in order to form a Utopian society where all animals would be treated equally. In the end, though, the pig named Napoleon emerged as a leader of the Rebellion and (along with his attack dogs) abused his power, colluded with the humans, and made sure pigs were treated better than other animals on the farm. In this final scene, animals fleeing the farm see that Napoleon and other pigs like him are no better than the humans who once oppressed them. As they spy Napoleon and his cohorts playing a game of poker with Mr. Pilkington and other humans, they can no longer distinguish the pig and human faces from one another.
(Animal Farm is well known as an allegory for Joseph Stalin's brand of communism that swept across Communist Russia and Eastern Europe following World War II, which did not end well.)
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