When Old Major calls the animals together to relate his strange dream and his philosophy of Animalism, they sit in audience seemingly at random:
First came the three dogs, Bluebell, Jessie, and Pincher, and then the pigs, who settled down in the straw immediately in front of the platform. The hens perched themselves on the window-sills, the pigeons fluttered up to the rafters, the sheep and cows lay down behind the pigs and began to chew the cud.
(Orwell, Animal Farm, msxnet.org)
Of their positioning, however, it is interesting to note that the dogs and pigs sit together, at the front, while the sheep and cows sit "behind the pigs." This shows how the relationship between the pigs and dogs will progress, with Napoleon training puppies to serve in his own private army. It also shows how the sheep and cows serve as a groupthink mob for the pigs, as they have little volition of their own, and usually simply repeat everything the pigs say. This is a subtle foreshadowing of their roles in the revolution, as well as the import they place on Old Major's words; the pigs listen intently, while the sheep and cows are content to mimic the pigs without putting forward independent thought.