Though the book may follow separate characters, there is usually only one character (or characters) that the book would revolve around.
Let's put it this way: you're writing a book from the view of one character, but you're telling the story of another character. The main character could possibly be the other character and not the one you're following.
But anyway, in the case of Animal Farm, being that I read it almost 10 years ago, I can't remember much. But I think I would consider the pig (or the one pig that seems to lead more) to be the main characters.