Orwell wrote Animal Farm during World War II, although he was unable to get it published until after the war. Through his experiences in the Spanish Civil War, Orwell came to despise the communist party, which he saw as petty and corrupt. His novel is an allegory of the rise of communism in Russia after the fall of the Tsars.
The term "comrade" is laden with many connotations. When applied to everyone, even those in positions of great power, it is meant to imply equality. When Old Major uses the term in chapter 1, this is how he intends it because his character represents Karl Marx, the German philosopher and economist whose theories spurred the development of workers' parties around the world.
Later in the story, the hypocrisy of using the term "comrade" to refer to Napoleon and the other pigs is, of course, very clear. The reader can see the corruption of the system of Animalism in parallel with the ridiculous nature of the term "comrade" as the pigs destroy all semblance of equality on the farm.