Mollie, the cart-horse, is not as thrilled about the revolution as the other animals. During the harvest, she finds ways to avoid work, complaining about fake pains and hiding. Eventually, she becomes friendly with a worker on another farm, and after interrogation, she vanishes from the farm. It is discovered that the worker was bribing her with sugar and pretty ribbons to leave the farm.

A fat red-faced man in check 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, so the pigeons said. None of the animals ever mentioned Mollie again.
(Orwell, Animal Farm, george-orwell.com)

Mollie does not want to live in perpetual labor for the other animals, but prefers to live in security even under a human master. Her ideals are contrary to the Utopian ideals of Animalism, but she "seems happy," and so is virtually erased from the collective mind. By "never mentioning her again," they can pretend that she never left what should be a wonderful life to live in "slavery." Her actions and her seeming happiness are contrary to everything that Old Major preached.