The old donkey Benjamin "gets wise" right away to what's going on at Animal Farm (the subversive take-over of the pigs), but he doesn't react or even comment on it. At first glance he seems apathetic but then his silence seems to be part of his strategy of simply staying alive. Benjamin has seen a lot of things and knows the simpler animals have lost from the very start. They are outwitted by the craftier and keener pigs, who manipulate them into subservience.
Boxer the workhorse is a member of this second category. He is sincere and full of good will, but too dull-witted to learn how to read or to observe the new hierarchy taking place. He repeats two slogans incessantly: "I will work harder" and "Napoleon is always right." He is the antipode of Benjamin, and they are indeed foils of each other.
Remember that Animal Farm is not simply a fable; it is also an allegory of the aftermath of the Russian Revolution and the rise of communism. The characters in the story represent individuals or types. Benjamin would be the intelligentsia stifled into silence through repression; Boxer represents the proletariat initially manipulated via propaganda, then through repression as well.