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Squealer could be considered the counterpart of Vyacheslav Molotov, who served as Stalin's Prime Minister during the 1930's. Molotov remained loyal to Stalin to the end, even being seen by some as the most likely successor to Stalin.
In a broader sense, though, Squealer didn't represent any specific person as much as he represented the whole propaganda machine created and exploited by Stalin. Squealer's primary contribution to the Revolution was communications - specifically, convincing the animals that their concerns were groundless and that everything was progressing exactly as was needed. Squealer's ability to turn everything around so that it gave the advantage to the pigs, specifically Napoleon, was his most important function.
No one believes more firmly than Comrade Napoleon that all animals are equal. He would be only too happy to let you make your decisions for yourselves. But sometimes you might make the wrong decisions, comrades, and then where should we be?
If Napoleon truly believed that all animals were equal, there would have been no reason to be concerned about allowing the animals to make decisions for themselves!