en.wikipedia.org/wiki/Boxer_(Animal_Farm)

This passage misstates Old Major's warning in Chapter 1 of Animal Farm. The novel says Jones would sell Boxer to the knacker, who would cut his throat and boil him down for the foxhounds. That describes Boxer being slaughtered and rendered for hound feed, not specifically made into glue.

A dictionary definition of knacker supports that reading: a knacker is a buyer of worn-out domestic animals or their carcasses for use especially as animal food or fertilizer.