Everything that happens in George Orwell's novel Animal Farm represents something; what does Mollie's departure signify?
It is true that George Orwell's Animal Farm is an allegory (he calls it a "fable") in which the places and characters are all representative of something real. This is a story primarily about the Russian Revolution: Manor/Animal Farm is Russia, and all of the characters are depictions of real people or groups of people who lived there.
Mollie is a white mare, and from the very beginning she does not seem to be committed to any cause but her own. Shortly after Old Major dies, the animals all gather to hear more about the idea of a rebellion (revolution) at some point in the future.
The stupidest questions of all were asked by Mollie, the white mare. The very first question she asked Snowball was: "Will there still be sugar after the Rebellion?"
She follows that question with another: will she still be able to wear ribbons in her mane?
Most of the other animals are not treated as well as Mollie is treated by Mr. and Mrs. Jones; we know she is one of their favorites because she is given sugar while many of the other animals are routinely mistreated. Clearly her sole interest in any changes on the farm are centered around maintaining her rather luxurious lifestyle.
Mollie is self-centered and demonstrates it regularly throughout the novel. Immediately after the rebellion, the animals walk through the farmhouse. They discover Mollie has gotten sidetracked; she is holding one of Mrs. Jones's ribbons against her shoulder and foolishly admiring herself in the mirror. When the animals are harvesting the crops, Mollie does not do her share of the work.
Nobody shirked--or almost nobody. Mollie, it was true, was not good at getting up in the mornings, and had a way of leaving work early on the ground that there was a stone in her hoof.
When the animals begin to learn how to read and write,
Mollie refused to learn any but the six letters which spelt her own name. She would form these very neatly out of pieces of twig, and would then decorate them with a flower or two and walk round them admiring them.
When Jones and the others come back to fight for the farm, Mollie spends the entire battle in the barn with her head buried in some hay. One day Clover sees a neighboring farmer petting Mollie's nose (a fact Mollie denies), and when Clover searches Mollie's stall, she finds "a little pile of lump sugar and several bunches of ribbon of different colours. Three days later Mollie disappeared."
It is obvious that Mollie cannot represent the common man, like Boxer, because there is nothing common about her; instead she is more connected to the leadership in power, Mr. Jones (Tsar Nicholas II). She is relatively disinterested in what happens on the farm unless it affects her, and she adores pretty things and delicacies such as sugar. Eventually she leaves and is later seen pulling the smart-looking carriage of a man who feeds her sugar and keeps her looking neat and pretty. She has found a new master.
In this novel, Mollie represents the aristocrats, many of whom were connected to Russian royalty, who were more concerned about themselves and maintaining their luxurious lifestyles than about anything else. In Russia, most of the aristocrats left the country during this time of unrest, violence, and deprivation, as well.