The seating arrangement shows two things:
It reveals the hierarchy of the animals, with the pigs escorted by the dogs (later to become Napoleon's secret police) to the very front, just in front of Old Major. (for the most part they are also, apart from Napoleon, the progeniture of the old boar.) At the other extreme, the chickens take their places on the windowsills and the pigeons, in the rafters, comparable to the third rate seats in the "slits" of a theatre. The other animals, according to their station, take their place in-between.
It displays the natural predisposition of the animals. Clover the mare, Boxer's companion, takes great care to protect the ducklings from her heavy hooves as she settles down, but they seem too nestle against her for comfort. Mollie, the absent-minded and narcissist white pony, prances in late (as if to be noticed), flaunting plaited ribbons in her mane and tail. Muriel the goat, Benjamin the donkey, the cat and Moses the crow keep an observant distance from the front platform (or are absent), as they are not part of this collective scene. The comportement of the animals is already an indication of the respective roles they will assume as events unfold.