What is significant about the pigs and the raven in Animal Farm?

In George Orwell's Animal Farm, the
types of animals that are chosen to play particular characters are based on the traits
that those animals embody.  After the animals take over the farm, even though all the
animals are supposed to be equal, a hierarchy develops and the pigs sit comfortably on
top.  The pigs Old Major, Napoleon, Snowball, and Squealer are intelligent animals who
provide guidance (albeit different types of "guidance") to the animals on the farm. 
Pigs are known to be intelligent farm animals, so Orwell exploits this trait when he
personifies the pigs in Animal Farm.  Similarly, Moses the raven is
a preacher in the story and delivers sermons to the other animals on the farm.  He
represents religion, and the other animals are placated by his words.  Ravens are known
for their croaking voices, so again this trait is exploited in the
novel.