In chapter 9, Boxer has a split hoof and it take a long time to heal. He does get somewhat better, but as the work progresses in building the windmill, he collapses from overexertion. When this happens, some of the animals tell Squealer. Squealer says that they will bring him to a human hospital to get him cured.
When the truck comes to take Boxer, Benjamin reads the sign on the truck and he realizes that Boxer is going to a slaughter house to be made into glue. The other animals scream. Here is what the text says:
‘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!'
A cry of horror burst from all the animals.
Later Squealer tries to tell the animals that the human hospital bought the truck from the knacker but did not change the sign. It was all a lie, of course.
What we see here is the tragedy of Boxer, the hardest working animal, who is killed when he can no longer produce.