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Mollie is stupid, vain, and materialistic. (It's a good thing she's pretty.) From the very beginning, we get hints that she's not going to last long in the rebellion: she comes late to Old Major's speech, and she "took a place near the front and began flirting her white mane, hoping to draw attention to the red ribbons it was plaited with" (1.4). The first thing she wants to know is, "Will there be sugar after the rebellion?" (2.3); the second thing she wants to know is whether she'll be allowed to wear ribbons.
Not that she waits for an answer: after the rebellion, the animals find her in the farmhouse, where "she had taken a piece of blue ribbon from Mrs. Jones's dressing-table, and was holding it against her shoulder and admiring herself in the glass in a very foolish manner" (2.18).
When the work begins, Mollie shows up late and leaves early. When there's fighting, she hides in her manger. And when life gets hard during the winter, she gets troublesome: "She was late for work every morning and excused herself by saying that she had overslept, and she complained of mysterious pains, though her appetite was excellent" (3.1). Clover eventually sees Mollie letting one of the neighborhood men pet her nose, and soon after Clover and some other animals discover sugar hidden in Mollie's manger.
In the end, Mollie runs off to be taken care of by humans—and we say, good riddance. (Or we would, if we didn't suspect that we'd end up doing the exact same thing in her situation. Shmoop looks super pretty in blue ribbons.) When Mollie runs off, the narrator notes that "none of the animals ever mentioned Mollie again" (5.7). She's nothing but a bad memory—a reminder that not everyone prefers life at Animal Farm.
Mollie is symbol for Russian middle class (bourgeois). They weren't exactly unfaithful to the Bolsheviks, but they weren't about to give up their iPhones and lattes—oops, we mean sugar and ribbons—even if it was supposed to be for their own good in the long run. When the Bolsheviks asked them to give up their luxuries, many of them abandoned the cause and fled to the West.
Of course, in retrospect, Mollie probably had the right idea. If she's going to be oppressed no matter what, she might as well get some ribbons for it.