Mollie runs away from the farm because she has never supported the rebellion. Mollie loved being under Mr. Jones' rule because she got plenty of sugar and she was allowed to wear ribbons in her hair. She got to be a pretty horse, all show- no work. Under the regime of the pigs Mollie was not only denied her sugar and ribbons, but she was now also required to work! She had trouble getting up early for work and she often liked to quit early. Finally in Chapter 5 Mollie is confronted by Clover about her laziness and her whereabouts during work. The confrontation drives Mollie to run away from the farm so that she could go somewhere where she could finally have sugar and ribbons.
Mollie is representative of those who are too vain and materialistic to care whether or not they are free. Mollie represents those people who would rather be led under a dictatorship than work to be liberated.