It is in Chapter 5 that we are told that Mollie is working less and less and putting less effort in, and then she disappears:
Three days later Mollie disappeared. For some weeks nothing was known of her whereabouts, then the pigeons reported that they had seen her on the other side of Willingdon. She was between the shafts of a smart dogcart painted red and black, which was standing outside a public-house. A fat red-faced man in cheek breeches and gaiters, who looked like a publican, was stroking her nose and feeding her with sugar. Her coat was newly clipped and she wore a scarlet ribbon round her forelock. She appeared to be enjoying herself, the pigeons said.
So Mollie exits from the novel as is never mentioned again.
Unlike Boxer, who always thinks of others, Mollie is a shallow materialist who cares nothing for the struggles of her fellow animals. her first appearance in the novel suggests her personality when she enters the meeting at the last moment, chewing sugar and sitting in the front so that the others will be able to admire the red ribbons she wears in her mane. Her only concerns about the revolution are ones prompted by her ego: When she asks Snowball if they will still have sugar and ribbons after the rebellion, she betrays the thoughts of old Major and reveals her vanity. She is lulled off the farm by the prospect of more material possessions that she could enjoy in an animal-governed world, marking her as one to whom politics and struggle mean nothing.
Mollie was indeed a materialistic animal, and she craved the attention from human beings, who would sent her gifts like ribbons, making her look pretty. She does not really care much about Animal farm, usually concerned about her appearance. She loves being taken care of, being pampered and groomed. She find it very hard to settle in Animal Farm as she misses wearing ribbons in her mane and also chewing sugar cubes in the past. She loves being admired by other fellow animals, but now it was impossible to get that sort of attention. So, when a "red-faced man" gave her a ribbon, she was cajoled into leaving the farm and into the trap of the human-governed farm, based on the material possessions that the humans have that the animals does not.