When Old Major delivered his speech which contained the idea for the animal revolution, one of the things he forecast was that, if human rule continued, Boxer would be sold to the knacker “the very day that those great muscles ... lose their power.” The irony is that the pigs, who were supposed to be better and kinder leaders of the animals had now done the same thing Old Major predicted the humans would do. This exemplifies the idea that the pigs have now become just as brutal and uncaring as the humans whom the pig replaced. The pigs now symbolize repression and greed just as the human had once symbolized the same characteristics.
In Animal Farm, Boxer, who is among the hardest working animals on the farm, toils to build the windmill until he collapses from exhaustion. His lung has simply given out, and the pigs send for the knacker, who turns horses into glue, while claiming that they are sending for a veterinarian to treat Boxer. When Benjamin reads the side of the knacker's truck and becomes alarmed, Squealer lies to the animals and tells them that the veterinarian was driving the knacker's truck (a knacker is a person who disposes of dead animals).
The knacker symbolizes the way in which the Soviet officials under Stalin and other leaders exhausted the workers through hard labor. Though the workers believed in the system, they were not well cared for when they could no longer work. Lenin said during the Russian Revolution, copying a New Testament saying, "He who does not work, neither shall he eat." Therefore, the Soviet system did not care for its elderly or for workers who became infirm.