To be completely accurate, throughout the book Animal Farm, the animal who is in charge who regularly robs the other animals of freedom and equality is Napoleon.
If your question is truly about Boxer, here are a few ideas that might relate. Boxer works so hard that he doesn't realize that he is constricting himself and others to a life of work. Because Napoleon knows that Boxer (who represents the Russian working class) "will work harder" Napoleon counts on this and ultimately promises items to purchasers later in the book. By being subservient to Napoleon and his rule, Boxer ensures a loss of freedom and equality for the rest of the animals.
Ultimately when Boxer is no longer able to serve, Napoleon has him sent to the "hospital"/knacker. Boxer's presence did always at least provide a protection of the rest of the animals, but when he is purged from the rest of the group forever, they no longer have him to stick up for them if he even would have used his strength to do so. This ensures they will never be free.