Pigs. Pigs are increasing with the help of Napoleon. He has spent much of his time on the farm siring new litters of pigs to raise on the farm. These sons and daughters of his are being educated by himself personally, to be groomed to "lead" the society. The things they learn in school are reading and writing and also social rules like other animals are to stand aside when they walk by.
The chickens and roosters are decreasing rapidly in number on the farm. They were told at first that for once in their lives they would get to see their young and raise them, but Napoleon made a deal with the humans to sell 400 eggs per week! This meant that not only would the hens not get to see the young, but they had to also produce more eggs than they had in the past. the hens who tried to revolt against this by laying their eggs from the rafters so that they would smash to the ground were executed. Napoleon was a leader whose source of power came from others' fear and this struck the fear into the hens to meet Napoleon's demands.