George Orwell certainly did put in a great deal of work to fill his novel Animal Farm with symbolism to illustrate his themes. We can see symbolic reasons behind his animal choices.
Orwell uses pigs to symbolize the leaders of what was actually the Bolshevik Revolution and the start of communism in Russia because pigs are actually known to be the most intelligent animals found on a farm, next to dogs. Studies have shown that pigs are able to use mirrors to their advantage and do all kinds of tricks, including "jump hoops, bow and stand, spin and make wordlike sounds on command, roll out rugs, herd sheep, close and open cages, play videogames with joysticks, and more" (Angier, "Pigs Prove to be Smart, if Not Vain"). Hence, it was most definitely appropriate for Orwell to symbolize Carl Marx, leader of Russia's communist revolution, as an old, wise pig named Old Major. What's more, pigs are also known to represent uncleanliness as can be seen in the fact that they are known to wallow in mud and eat any slop and in the fact that pigs are forbidden cuisine in many religions like Judaism. Uncleanliness can also be likened to moral depravity; hence, it's also very fitting for Orwell to use the pig Napoleon to symbolize Stalin, the pig Snowball to symbolize Trotsky, and other pigs to represent other aspects of Stalin's regime.
Boxer is a horse whose motto is always "I will work harder." Boxer symbolizes the Russian people in general. Horses are known for their loyalty and their willingness to work, making horses a perfect way to symbolize the willing people of Russia.
Benjamin is a donkey who always questions the rightness of the pigs and refuses to do as they say because he sees any action as being futile; no matter what anyone does, "hunger, hardship, and disappointment" will always be, as he says, "the unalterable law of life" (Ch. X). What's more, Benjamin remembers every detail of the farm's difficult life. Benjamin symbolizes the older, cynical Russian citizens who did not believe communism would benefit society and even saw communism's evil side. Using a donkey is a perfect way to symbolize cynical citizens because donkeys are known to be stubborn, have good memories, and are unwilling to do anything that's not safe (Mike's Donkeys, "Donkey Facts").