As others have mentioned, the pigs have more or less cornered the market on selfishness—but not entirely. Mollie shows her selfishness when she shirks her duty by getting up late and leaving work early. She will only learn the letters of her name and no others. She hides lumps of sugar and ribbons for herself. She finally goes back to pulling a cart for a human. Mollie is interested in her own well-being more than helping others or working for a cause greater than her own comfort.
The cat disappears when it is time for work and then shows up for meals, also not pulling her weight.
But the pigs, by far, are the most selfish creatures on Animal Farm. They care only about their own well-being and are willing to work the other animals to the bone to live in comfort. Even early on, they take milk and a windfall of apples for themselves, not sharing with the other animals. Squealer, of course, lies vigorously, denying that the pigs are selfish:
You do not imagine, I hope, that we pigs are doing...
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