Most of the overt symbolism in Animal Farm comes from the representation of real-world events and people in the form of animals on a farm. For example, Boxer is representative of the working class, hard workers who do not question authority but instead work harder in the hopes that their work will be recognized and rewarded. This extends to the plot, which shows that in fact, hard work is not rewarded -- Boxer is sold for the price of whiskey when he can't work anymore.

Another good example is Squealer, who represents propaganda; by using circular logic, lies, and semantics, Squealer is able to twist the meaning of events and facts.

"Many of us actually dislike milk and apples. I dislike them myself... We pigs are brainworkers. The whole management and organisation of this farm depend on us... It is for your sake that we drink that milk and eat those apples. Do you know what would happen if we pigs failed in our duty? Jones would come back!"
(Orwell, Animal Farm, msxnet.org)

These small lies become larger and larger until Squealer is quoting from imaginary reports that "prove" how prosperous the farm is, and how little work the animals perform, even as the evidence of their eyes disproves it. Working in conjunction with Napoleon's dogs (the military) Squealer is instrumental in convincing the animals that they are actually being oppressed for their own good. If they refuse to submit to Napoleon, Jones will return -- even though that would be an improvement in their lives.