In Animal Farm, the animals are not all equal and this is made clear from the beginning of the story when the plans for the Rebellion begin. The pigs, for example, rise to prominence as they assume responsibility for planning the Rebellion. Once Mr. Jones is overthrown, the pigs' dominance continues: Napoleon steals the milk and apples, for instance, so that the pigs can eat better rations.
Similarly, at the beginning of Chapter Three, it is revealed that the pigs do not do any of the physical work required for the maintenance of the farm:
"With their superior knowledge it was natural that they should assume the leadership."
The pigs, therefore, exploit the idea that they are more intelligent than the other animals. They then use this advantage to enjoy a better standard of living on the farm and to assume total control, as we see most clearly through the character of Napoleon.
Animal Farm more specifically has two classes: elite and proletariat. The pigs strategize and act as propagandists (Squealer). Boxer and Clover, both cart horses, represent the proletariat most dramatically, but all of the animals are in this class. There is no middle class because class distinctions are based on race being a pig or not being a pig. Dogs are an exception, for some are trained to be fierce to protect the pigs in power. The dogs represent the KGB of the Communist regime in Russia, but they are hardly middle class because Orwell treats them as mindless killing machines. It is true that Mollie, loving ribbons and sugar, does not accept the ideologies of the pigs as readily as the rest, but after she is caught being petted by a human, she quietly leaves the farm. Animal farm has no place for an animal with bourgeois tastes or refinements. Finally, some animals are more enthusiastic than others in accepting the new regime, but they are nevertheless part of it. The raven is the one exception. He flies to other places with reports of a beautiful land--perhaps he represents some aspect of religion.