The pigs are the most intelligent animals because they take control.
An inference is a deduction based on the evidence. The pigs are clearly very intelligent, and they use this intelligence to take charge of the situation and also to take advantage of the weaker animals.
The work of teaching and organising the others fell naturally upon the pigs, who were generally recognised as being the cleverest of the animals. (Ch. 2)
Of course it is a pig, Old Major, who introduces the whole idea of the rebellion. The other pigs, mainly Napoleon and Snowball, make it happen. The other animals go along with it in varying degrees. Benjamin the donkey seems fairly intelligent. He is pessimistic, but knows the score. He can also read as well as the pigs. The horses are loyal but not particularly intelligent.
These two had great difficulty in thinking anything out for themselves, but having once accepted the pigs as their teachers, they absorbed everything that they were told, and passed it on to the other animals by simple arguments. (Ch. 2)
The stupidest animals seem to be the chickens and the sheep. They are stubborn but not bright, and the tenets of Animalism have to be reduced to “four legs good, two legs bad” for them. The pigs teach themselves to read and write and manage to instruct a few other animals, including the dogs, but most animals can’t really learn.
Another way we can tell that the sheep and chickens are not very intelligent is that they falsely confess. They have been carried away by sensationalism. The dogs can kill them because they are smart enough to know who is in charge now, and they just switch their loyalties from people to pigs. This is why Napoleon specially trained the litter of puppies to be his secret police.