The greatest tragedy of the book is what happens to Boxer. There are two famous mottos of Boxer. He says: "I must work harder," and "Napoleon is always right."
These two quotes shows what Boxer is about. He is a hard working animal. The more difficult the work, the more he will dig deep inside and find the strength to do it. He is a work horse, literally. Second, he is loyal. He is one who truly believes in the revolution. In fact, he is the most loyal animal on the farm.
In view of this, we would expect him to be honored, but he is not. When he grows ill and sick, the pigs do not take care of him. They used him in the past and they will use him now. They sell him to the knackers and they will boil his body and make glue. He is completely expendable and nothing of his labor remains. It is as if he never lived. By far, he is the most tragic, because he was sincere in all that he did and completely taken advantage of.
Boxer is taken to the glue factory after hurting himself working on the windmill. Boxer knows that something is wrong with his lungs and that his strength has completely left him.
The pigs tell the other animals that he is being taken to the hospital to be treated, but Benjamin reads the side of the van and tells the truth. Of course, Squealer is able to persuade the animals otherwise after Boxer is gone.
Boxer's death is a huge blow to the animals for many reasons. He was by far the hardest worker on the farm, especially on the windmill. He was a source of strength for the other animals as rations were reduced and they started to be constantly hungry. Benjamin and Clover in particular missed his company. Napoleon also loses one of the few animals that blindly followed his lead without asking questions.