The sheep follow the herd thinking more than most animals so they are apt to represent these.
Explanation:
In the novel "Animal Farm" the sheep are the common people who will parrot anything they are told is true by their authority and will not question it.
Thus, they are well played by the sheep who are known to be herd animals.
Sheep usually stay in their herd and follow their herd even to imminent deaths as their minds are not designed to think for themselves.
They also do not exist in the wild and like to have a master of the herd for themselves.
