As noted by the previous educator, the number of pigs increases in Chapter Nine of Animal Farm, while the number of chickens decreases. What is significant about this population change is that it is indicative of Napoleon's rule over the farm. The pigs, for example, are able to flourish because they are 'more equal' than the other animals. As we see in Chapter Nine, their rations are not reduced, they live in the farmhouse and receive a special education from Napoleon. In other words, the pigs are thriving.
While the pigs receive the best of everything, the other animals suffer considerably. In this chapter, their rations are reduced and they are "often hungry and often cold." In all, life is "harsh and bare," especially for the hens, who are now expected to lay eggs more than ever. We also learn in this chapter that not a single animal has been granted their retirement.
This chapter, then, suggests that the pigs are thriving at the expense of others. Napoleon is breaking the seven commandments, specifically the idea that all animals are equal, and this is directly affecting the population of Animal Farm.
Pigs are the obvious animals that are increasing thanks to Napoleon.
"In the autumn the four sows had all littered about simultaneously, producing thirty-one young pigs between them."
The species that are decreasing are horses and chickens. There were only three horses on the farm and in Chapter 9, Boxer the most loyal and steadfast horse is worn out, he hurts his lung working too hard. He is looking forward to retirement, but instead Napoleon has him sent to the Knacker's to be slaughtered. Chickens have been decreasing ever since Napoleon demanded that they surrender all their eggs to be sold so that the farm would have money for supplies.