Why does Orwell have the animals refer to each other as “Comrades”?

Animal Farm is a political allegory on life in the Soviet Union. Animalism is Communism, Napoleon the pig is Stalin, the shire-horses are the Soviet workers, Snowball is Trotsky, and so on. In the Soviet Union, people were encouraged to call each other "comrade." This is a common expression in the political Left; it is supposed to express friendship and solidarity. In the Soviet Union, it was also supposed to be an expression of equality; everyone in the country was formally equal, from Comrade Stalin to the comrade next door. In reality, however, some people were more equal than others, with the political elite enjoying a much higher standard of living than ordinary working people in what was supposed to be a worker's paradise.
In Animal Farm, as in the Soviet Union, the term "comrade" is cynically used as a means of covering up the glaring inequalities of the political and economic system. Like Stalin and the Bolsheviks, Napoleon shamelessly manipulates language to create an alternative reality. And in this alternative reality, all the animals on the farm are equal; they are all "comrades," engaged in a common struggle to build an Animalist utopia. But this crude manipulation of language simply masks the reality of a brutal, inefficient order, in which the pigs are firmly in charge of the farm and all the other animals are little more than slaves to be used, abused, and exploited.