What eventually happens to Boxer and why?

Boxer’s split hoof, an injury from the battle, taxes him; still he will not let it deter him from rebuilding the windmill before he reaches retirement age. Boxer and the other animals work feverishly to complete their tasks, which now include building the schoolhouse for the young pigs. One day, Boxer overworks himself so much that he collapses, unable to get up. In his sickly state, he expresses a wish to retire early along with Benjamin. The animals fetch Squealer, who relays Napoleon’s decision to send Boxer to the veterinary hospital in Willingdon.

Over the next two days, Boxer lies in his stall and takes doses from “a large bottle of pink medicine” that the pigs send from the farmhouse. He expresses his wish to spend his final years learning the rest of the alphabet. One afternoon, a van comes to take Boxer away. It has “lettering on its side and a sly-looking man in a low-crowned bowler hat sitting on the driver’s seat.” The hopeful animals wish Boxer goodbye, but Benjamin breaks their revelry by reading the lettering on the side of the van: “Alfred Simmons, Horse Slaughterer and Glue Boiler, Willingdon. Dealer in Hides and Bone-Meal. Kennels Supplied” (123). The animals panic and try to get Boxer to escape. He tries to get out of the van, but he has grown too weak to break the door. The animals try to appeal to the horses drawing the van, but they do not understand the situation.