Animal Farm is an allegory of the Russian Revolution and the subsequent policies of the U.S.S.R. Napoleon represents Joseph Stalin and, less directly, Napoleon Bonaparte, the latter in order to show how power corrupts. Snowball represents Leon Trotsky. And Old Major represents Karl Marx. Stalin (Napoleon) and Trotsky (Snowball) played significant roles in the Russian revolution. Ultimately, Stalin made a non-aggressive pact with Hitler (Mr. Pilkington) which Trotsky did not agree with. As a result of this and other disagreements, Stalin had Trotsky removed from power and deported; just as Napoleon did with Snowball.
Karl Marx (Old Major) was a German philosopher most famous for writing The Communist Manifesto and Das Kapital. In Marx's philosophy of history, called historical materialism, capitalism would give way to socialism (or communism) with a Worker's Revolution. This had tremendous impact on Stalin, Trotsky and other socialist/communist revolutionaries in Russia. So, Old Major represents this influence on Napoleon, Snowball and the other animals and Old Major represents the founder of the philosophy upon which the animal revolution was based.
George Orwell fashioned Old Major on two historical figures: Karl Marx, the German philosopher and political economist, and Vladimir Ulyanov (political alias Lenin), the Russian revolutionary leader.
Having lived as a homeless person himself in order to experience life at its lowest level, Orwell eked out a living as a dishwasher, and his contact with unemployed men turned him toward socialism. He also grew disturbed about the spread of dictatorships and he became disillusioned with Communism. Because of this disillusionment, Orwell wrote Animal Farm.
Old Major is devoted to the overthrow of the capitalist society of Mr. Jones and the liberation of the proletariat, represented by the farm animals. He encourages the animals to eliminate their real enemy, who is man:
Remove Man from the scene, and the root cause of hunger and overwork is abolished for ever.
Like Lenin, Old Major calls for "Rebellion" from the "tyranny of humans" because humans consider only their own desires and needs. "Man serves the interest of no creature except himself." He calls for unity among the animals and perfect comradeship. He instructs the animals that they must never tyrannize other animals or kill any other animal: "All animals are equal." He teaches the other animals a "stirring tune" called "Beasts of England."