George Orwell's Animal Farm is an allegorical depiction of life under a totalitarian regime, first under the control of a disgruntled farmer and then under the control of those who sought, in the name of liberty and equality, to replace the deposed farmer. Orwell's novel was inspired by the excesses of Bolshevik (communist) rule in Russia following the revolutions of 1917. As readers of Lenin, Trotsky, and myriad other communist leaders know, the use of the word "comrade" was a common title for those who shared their political and ideological leanings. While the word itself predates the development of communist or socialist ideologies, its application by theoreticians and others among communist ranks was routine, as when Trotsky repeatedly referenced Lenin as "Comrade Lenin."
In Animal Farm, the animals are motivated to rebel against the farmer by Old Major, a venerated boar who gathers his fellow animals together to address them as a prelude to action:
I do not think, comrades, that I shall be with you for many months longer, and before I die, I feel it my duty to pass on to you such wisdom as I have acquired.
Major continues to employ the word "comrade" throughout his address. The word becomes a part of the new order's vernacular, with animals addressing and referencing each other accordingly, as when Snowball, Napolean, and others rally their fellow revolutionaries.
"Comrade" does not have to apply solely to practitioners of totalitarian political philosophies, but its use became identified with communism by virtue of the writings and statements of leaders of communist movements.