Boxer split his hoof fighting the people during the Battle of the Windmill.
The humans are not happy leaving Animal Farm to the animals. After losing the Battle of the Cowshed, they regroup and come back again in what the animals end up calling the Battle of the Windmill, because the windmill is destroyed. In this battle, Boxer is injured badly. He fights valiantly, attacking many men by bashing their heads in with his hooves.
His knees were bleeding, he had lost a shoe and split his hoof, and a dozen pellets had lodged themselves in his hind leg. (Ch. 8)
After the battle, Boxer takes a long time to heal. The pigs had previously announced that older animals would retire, but that is not what happens to Boxer. Even though he is the one animal who always supported Napoleon, and always said he would work harder and Napoleon was always right, Napoleon does not support him. The pigs tell the animals Boxer is going to get treatment, but Benjamin knows better.
"Fools! Fools!" shouted Benjamin, prancing round them and stamping the earth with his small hoofs. "Fools! Do you not see what is written on the side of that van?"…
" '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)
This is proof that the pigs are no different than the people. They are opportunistic and only out for profit. Boxer is no longer useful because he can’t work, so they are going to get what little money out of him they can. Even though he is a fellow animal, they are selling him to a butcher. This is just what Jones would have done.
This incident demonstrates that the pigs really are no different from the humans in how they manage the farm, except that they might be colder and more heartless. They take advantage of their own kind. The people see animals as things to own, but the pigs seem to see the other animals the same way. Isn’t it worse when you betray your own? This is why by the end of the book, the animals can no longer tell the people from the pigs.