Clover is a female horse who is Boxer's close friend. She symbolizes the women of the Revolution. She is a motherly figure who is caring and provides comfort to the other animals. She is a firm believer in Napoleon, but is sometimes suspicious.
Clover is described as being a bit dumpy, having never gotten her figure back after her fourth foal. She is a cart horse, not a sleek pretty horse like Mollie.
Although Clover does not represent any person in history in particular, she does represent the working class women in Russia. They worked as hard as the men for the same empty promises of Communism.
My take on it is that Clover ultimately is not matched with Boxer, despite her devotion to him, but rather with Benjamin. It is not accidental that the book begins with her and ends with her. Outside the narrow confines of the classes in the revolution, I think Clover represents "Mother Russia," in much the same way that Benjamin is "Father Russia," perhaps someone like Leo Tolstoy. She represents a decency that lives on despite the rotten things that happen around her, and offers a hope for something better in the future when the evils that pervade Animal Farm have been swept away. I know that Clover is not just inserted into the book to give it another character. There is not a single word or sentence in this book that is not a part of the allegory as a whole.