The previous post has much truth... I'm going to give you some pop culture ideas.
Napoleon reflects a pig. Mom calls you a pig when you eat in a way that is either fast or messy or both. We call extra spending that doesn't need to occur from our government pork-barrel spending. People even refer to our policing authorities as 'pigs' sometimes. Think about the ways Napoleon was selfish or an authority in Animal Farm.
Benjamin, the donkey, is a doubter. The dissenter in a group is often called a donkey, or a more vulgar version of that word. That dissenter is always there, always causing strife.
Boxer is horsepower. We refer to our engines and their abilities to work and use energy in terms of horsepower - brute strength.
Dog is a man's best friend. Man likes this animal because of its moldability. Man controls this world, and our ability to train dogs is reflection of that.
Sheep are dumb. The bible uses this word image between God and men to point out the vast difference between the two. Sheep would watch each other fall off a cliff and still walk off that cliff. Watch how drastically sheep follow in Animal Farm.
Boxer is a hard worker. He is a horse that is 18 hands high. He is not of exceptional intelligence, but is loyal. Horses are measured in hands and are often used on farms as an extended labor resource. Boxer was used to pulling things for his farmer. He continues to do these things after the animals take over.
Napoleon is a coward. He is a large fierce looking boar (pig). He is highly intelligent. Napoleon takes nine puppies and raises them himself. Pigs will nurture other animals and feed them; however, these are usually female pigs. Napoleon had a plan in mind. Napoleon has a violent nature. Pigs have been known to eat their own young.
Benjamin is a very intelligent donkey. He is also the oldest among the animals and cynical. In animal behavior he demonstrates loyalty to the horse Boxer. Donkeys and horses often get along well in pastures. Donkeys can be intelligent and stubborn in nature. They tend to have long lives. One of his faults is that he tends not to act on things. Donkey's can be stubborn refusing to comply with farmers expectations. After the rebellion he continues to perform his work in a slow manner just as he had done when the farmer was present.
Sheep have a tendency to form in flocks and to follow as guided. The sheep in Orwell's story are much the same. They are a collective group of followers. They are easily influenced by the will of others.
Dogs in real life become a reflection of the manner in which they are raised. Fro example, a pit bull that is raised to fight will fight, a lap dog will lie calmly on a person's lap, and a hunting dog trained to hunt, hunts. Napoleon has trained the dogs in the story to use their power to control the sheep. They are what Napoleon has trained them to be.