When the animals revolt, it is assumed that all the animals will participate in working for the common good, as Old Major wanted. However, the pigs are smarter than the other animals, and use their intelligence to avoid working:
The pigs did not actually work, but directed and supervised the others. With their superior knowledge it was natural that they should assume the leadership.
(Orwell, Animal Farm, george-orwell.org)
At first, this can be seen as performing the role of supervisor; without their knowledge of human tools and reading, the other animals would have had a very hard time learning to use tools or working the harvest. However, even from the beginning, it is clear that the pigs are gaming the system; they keep the milk from the cows for themselves, on the pretext of needing extra nutrition for thinking. Eventually, they move into the role of master even more than Farmer Jones ever had, completing the cycle of a failed revolution.