Boxer is sent to the knackers to be slaughtered. It is ironic because it was one of the abuses Old Major accused humans of perpetrating against animals.
Old Major sells the animals on a dream of equality and opportunity. In his vision, no animals will ever be held hostage to humans. Animals will work together to create an atmosphere of cooperation and comfort.
It does not take long for the dream to sour. As soon as the animals kick the humans off the farm, the pigs take charge. From that point on, they slowly gain more and more power at the expense of the other animals.
One of the promises made to the animals is that once they were too old to work, they would be able to retire. Humans would have killed them, but in Animal Farm they would be blessed with a peaceful existence.
As yet no animal had actually retired on pension, but of late the subject had been discussed more and more. Now that the small field beyond the orchard had been set aside for barley, it was rumoured that a corner of the large pasture was to be fenced off and turned into a grazing−ground for superannuated animals. (Ch. 9)
Boxer very much wanted to finish the windmill before he retired. He was one of the hardest working animals on the farm, and very loyal to Napoleon. His two favorite expressions were “Napoleon is always right” and “I will work harder.” He had no doubts about his leader. Unfortunately, when Boxer injured himself on the job and was no longer useful, he was not sent into retirement. He was sold for a profit. Benjamin was able to read the words on the side of the truck.
" 'Alfred Simmonds, Horse Slaughterer and Glue Boiler, Willingdon. Dealer in Hides and Bone−Meal. Kennels Supplied.' Do you not understand what that means? They are taking Boxer to the knacker's! " (Ch. 9)
The fact that Napoleon sold Boxer to the slaughterhouse for profit is ironic because Boxer was his strongest supporter and because Old Major promised the animals a better life. He told them that humans exploited them, and that they would be protected from this exploitation on Animal Farm. This dream might have been real for Old Major, but it never was real for Napoleon. He used the entire rebellion as an opportunity to gain power for himself. He did not actually care about the animals.