As a group, the pigs are undoubtedly the cleverest animals. They show this in the way that they take charge of the initial revolution against Mr Jones, masterminding the campaign against him. Thereafter they keep control over the other animals. Snowball in particular stands out as a pig of exceptional ability, not least in a military sense, although he finds himself pushed out of power by his rival Napoleon who is helped out by another pig, Squealer. Napoleon is not clever in the same sense as Snowball, who has a brilliant mind, but he is very clever in the way that he manipulates his way to supreme power. The pigs, largely because of this manipulative intelligence, manage to keep all the other animals down.
There are other individual animals that are intelligent but they are not able, or (in the case of Benjamin, the old donkey) not willing to work together as a group. If they had been, they might have been able to overthrow the pigs instead of letting themselves be oppressed by them.