Why do you think Orwell decided to represent the “proletariat” with the animal he chose?

It's generally thought that Orwell uses the ignorant and docile but hardworking horse, Boxer, to represent the Russian proletariat, or working class, in Animal Farm. The main reason for this is that the horses are incredibly strong and do most of the work on the farm but derive relatively little benefit. This is especially true in comparison to the pigs, who are lazy but all-powerful.
Though physically strong, Boxer is simpleminded and naive; he believes implicitly in the outrageous lies peddled by Squealer about the genius of Napoleon and the revolution's massive achievements. If there are chronic shortages on the farm, then it's not because there's something wrong with the system; it's because the animals aren't pulling their weight. So Boxer resolves to work harder for the success of the Animalist revolution. His self-criticism perfectly serves the needs of Napoleon's regime as it diverts attention from those really responsible for the problems at Manor Farm.
It's clear from examining the character of Boxer that Orwell doesn't have a particularly high opinion of the Soviet proletariat. Although the Russian Revolution was ostensibly carried out in the name of the workers, Orwell seems to suggest that they've been cheated—brutalized, exploited, and repressed by a Communist regime notably short on authentic working-class members. What Orwell finds particularly exasperating is the way that the workers, like Boxer, remain docile and uncomplaining in the face of extreme hardship and privation, while at the same time being repressed by those supposedly acting in their best interests.
Yet Orwell pays fulsome tribute to the Soviet proletariat in his depiction of the Battle of the Windmill, which is an allegory of the USSR's fight against Nazi Germany. During the battle, Boxer fights valiantly against Frederick and his men, playing a leading role in sending them packing from the farm. But at the end of the conflict, even he realizes that this was a Pyrrhic victory. After all that chaos and bloodshed, the animals have simply won back what they already had.
Once again, the parallels with the Soviet Union are obvious. World War II cost the lives of over twenty million people in the USSR, and although the Soviets eventually prevailed, the country was utterly devastated. What's more, the dictatorship of Stalin—just like that of Napoleon at Manor Farm—continued on relentlessly, even more brutal and oppressive than before. And still the proletariat resolved to work much harder.