The knacker represents the ultimate end for all of the animals, but specifically for horses like Boxer. It is symbolic of the last stop for all animals. When animals can no longer be of use, specifically horses, it is seen as "the end." Old Major is the first one to use the term as representative of how humans exploit animals until the end. His argument in the opening of the novel is that once the humans take all they can from the animals and the animals are no longer of use, the humans will send animals like Boxer to the knacker. Old Major deliberately uses this image to shock the other animals for no one would ever see the mammoth strength of Boxer as something that could ever end and to see someone like Boxer reduced to the knacker is something that causes consternation in all of the animals. The knacker is that end that all animals fear but also represents how humans use and abuse the animals for everything and anything. The knacker is relevant to the development of the story for when Boxer's lung is punctured and when he is unable to work, Napoleon works out a deal where Boxer is taken to the knacker without anyone knowing it until too late. In this, the knacker is seen, but not recognized, and is an example of the cruelty of the pigs. The use of the knacker in Boxer's death is representative of Orwell's point that political orders do terrible in the name of expediency and consolidating their own power. In this instance, it is the pigs, not the humans, that use the knacker against one of their own.