Why does Orwell choose to call Boxer and Clover the pigs' most faithful disciples?
Considering that Animal Farm is an allegory (or "Fairy Story" to use Orwell's label at the beginning of the book) for the Russian Revolution, each of the animals represents a real-life human. Sometimes, as in the case of Napoleon representing Josef Stalin, these correlations are to a specific historical figure. However, in the case of the horses, they represent a group of people- the Soviet working class.
Orwell is suggesting that the horses (working class) were strong and fiercely loyal, though perhaps not as educated or as intelligent as some of the other trades of classes of citizens (the pigs, specifically) which allowed the horses to be easily manipulated and literally worked to death for the benefit of their leaders, the pigs.
"Boxer was the admiration of everybody. He had been a hard worker even in Jones's time, but now he seemed more like three horses than one; there were days when the entire work of the farm seemed to rest on his mighty shoulders. From morning to night he was pushing and pulling, always at the spot where the work was hardest. He had made an arrangement with one of the cockerels to call him in the mornings half an hour earlier than anyone else, and would put in some volunteer labour at whatever seemed to be most needed, before the regular day's work began. His answer to every problem, very setback, was `I will work harder!' which he had adopted as his personal motto" (11-12).
And later, Boxer adopts a second motto to show his deep allegiance to the leaders of Animalism, to which he has devoted himself:
"Boxer, who had now had time to think things over, voiced the general feeling by saying: `If Comrade Napoleon says it, it must be right.' And from then on he adopted the maxim, `Napoleon is always right,' in addition to his private motto of `I will work harder'" (Orwell 22).
Napoleon is able to use Boxer's fierce loyalty against him by relying on Boxer's work ethic and strength to complete great tasks like building the windmill, and when Boxer finally collapses, rather than allowing him to retire to a field as promised, Napoleon sells Boxer to the "knacker" (glue-maker) for a case of whiskey.
This literal selling out in the novel metaphorically represents how the Russian Revolution was fought and the Soviet Union was built on the backs of the working class who did not receive exactly what they were promised by Stalin and Soviet leaders.