Just finished Watership Down, by Richard Adams. Yes, yes, I know most people read it in high school. What can I say, our Grade 11 English class was deconstructing Pink Floyd The Wall, okay?
Anyway, Adams did a marvelous job of building not just a believable group of characters, but a complete civilization, including class structures, political ideologies, even religious dogmae. Not unusual for a work of fiction -- Tolkien's Middle Earth and Auel's Ayla & the Clan are two examples -- but I can't recall this kind of detail in a story focussed on animals. Animal Farm, maybe?
Feel free to post replies with any other books you've read with such invented animal ideology. (Please, no science fiction or what today is called 'fantasy.' I'm not looking for exhaustive lists of the deities worshipped by the half-men, half-crab creatures of Rigel 4. I want takes on animal philosophy.)