Work Hard, Work Harder
Boxer is the horse who is constantly working for Napoleon, even though he is constantly at work, he receives almost nothing in return. He splits his hoofs daily and still receives no credit for his work.
Boxer's Allegorical Purpose
In case you haven't already figured out, Boxer is intended to represent the Russian Working Class during Stalin's reign. People were often forced to slave hard for hours, and received almost nothing but scraps of food. They often died of starvation or overworking their bodies. This is similar to how Boxer is constantly working and splitting his hooves and receiving little to no food in the end is taken to the glue factory to be slaughtered.