Pigs are intelligent animals, but very brutish and strong; in fact, they are easily cannibalistic.  So, the use of the pig as the leaders is appropriate, espcially in the brute force and cruelty designed for Napoleon.  Competition among boar hogs is certainly a fact of life.

Boxer is very docile as he would be, having been trained strictly for heavy work. And, the dogs as enforcers is also realistic.  The sheep who bleet the slogans certainly represent those people who spout the conventional wisdom of the day.  And, the vain Mollie who is easily lured by pleasure is also realistic.

Most farm animals are not able to defend themselves except by the large size.  The dogs, however, have sharp teeth and claws.  They make a natural security force.  This is why Napoleon takes them when they are puppies.

Pigs are intelligent animals, and in Orwell's story, they develop Animalism and assume the leadership of the revolution and the farm. On the other hand, they are associated with gluttony, and so their corruption and greed at the end of the book is not surprising. Boxer is an old draft horse, an ideal animal to symbolize hardworking peasants. Benjamin, the donkey, is stubborn, and represents a cynic who questions whether the rebellion will ever get better. The sheep represent a herd mentality that in part enables the pigs to maintain their power.