In Chapter 8 the animals fight off an invasion by Frederick and his men. During the battle, Boxer sustains several injuries, including "a dozen pellets … lodged … in his hind leg." A little later in the story, in chapter 9, this injury to his hind leg causes him to fall while he is dragging "a load of stone to the windmill."
A few days later the pigs arrange to have Boxer taken away. They tell the other animals that they are sending him to a veterinary surgeon, but in reality they are sending him to the knacker. A knacker is a person who disposes of ill or diseased animals, and in the process renders the bones and fat of the animal into various commercial products, such as tallow, glue, and soap. This, unfortunately, is Boxer's fate.
The animals realize too late that the van taking Boxer away belongs to the knacker and not to a veterinary surgeon. When they realize what is happening, they are horrified and afraid for their friend. They try to shout at the horses who are pulling the van, but the horses are too stupid to realize that they are taking a fellow horse to his death. Three days later Squealer tells the other animals that Boxer has died peacefully in a comfortable hospital bed, and the animals believe him.
At the end of chapter 9, a van delivers "a large wooden crate" to the farm, and later the same night the animals hear "uproarious singing" coming from the farmhouse where the pigs live. The animals discover, the next day, that "from somewhere or other the pigs had acquired the money to buy themselves another case of whisky." Although the animals remain completely oblivious to what is really happening, the reader is given to understand that the pigs bought the case of whisky from the money that they would have been given by the knacker, in return for Boxer.