This is a good question. In this chapter, Mollie begins to act "irresponsibly." She arrives to work late and even when she works, her heart is not into it. She also begins to talk to other men from other farms. She even takes treats from them. All of this is serious, because she is breaking the rules that have been set forth in Animal Farm.
Eventually she leaves; she is led away by a man who strokes her head and gives her sugar. Now she pulls his carriage. The pigeons who see these things report them to the animals on the farm.
The animals, according to the text, never mention her again. They move on. Here is what the text says:
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.