Boxer represents the working class, who unquestioningly accept their downtrodden position in society. Never doubting that the farm's leaders are right and have his best interests in mind, Boxer labors tirelessly in the belief that if he works hard enough, he will eventually get ahead.
Boxer is a truly good person at heart. As demonstrated by his remorse when he stuns the stableboy with his hooves during the Battle of Cowshed, Boxer has empathy for others and a great respect for life. Boxer is not terribly intelligent and is not a deep thinker; he is the type of character who does what he is told, submitting to authority and repeating party slogans without question. The windmill, the great triumph of the Animal Farm dystopia, is built upon his labor, but Boxer is not able to enjoy its benefits. His long hours of toil and deprivation take a toll on his health, and when he is unable to work anymore, he is sent by the very institutions he so faithfully served to the slaughterer.
Boxer, the hard working, loyal horse who stands by Napoleon and believes in his lies is a representation of the individual who accepted the propaganda from the leaders in Russia who told the people that life would be better for everyone after the Czar was gone.
After the Czar was gone, Lenin tried to institute a classless society, where everyone was equal. Like in Animal Farm, when the animals take over the farm, getting rid of Mr. Jones, they believe that they will all benefit from the new leadership. And, just like in Russia, the new leadership, Lenin, then Stalin, betray the people and their lives got harder, not better and the small elite class or rulers that emerged gained power and lived luxurious lives, while the peasants struggled to survive.
Just like in Russia, when Stalin staged the Great Purge, and millions of innocent people were executed, tortured or sent to Siberia, Boxer, is betrayed by Napoleon. No animal is safe on the farm, clearly, Boxer, who was so loyal and worked so hard and was always willing to work harder, should be rewarded for his life of labor.
He is therefore, a symbol of the baseless cruelty and indifference of Napoleon, who does not recognize the value of the devotion that he demanded from the animals and discards Boxer when he can no longer work.
Clearly, absolute obedience does not protect you, all promises are lies, you have no individual worth, that is the message.
The name “Boxer” may be referring to the Boxer Rebellion in China, which occurred in the latter part of the 19th century, when the peasants revolted against the foreigners that were controlling and destroying the Chinese economy. Orwell would be making an ironic commentary, as Boxer the horse continued to labor longer and harder for the interests of the ruling class, ultimately to his own detriment.