Nobody has mentioned
Watership Down yet?
One trick that really makes
Watership Down click is its sense of vocabulary. Cars, for example (and tractors, really all motor vehicles) are
hrududu, and dogs are
rowf, because... well, that's how they sound, and that's how the rabbits relate to them. It's very easy to get carried away with this and fall into the
"Call a Rabbit a Smeerp" phenomenon, but done carefully it can provide an interesting look into the way your animals think and perceive the world around them.
(( As an aside, in
Watership Down, rabbits speak freely to each other in a lapine language, but inter-species talk happens in something called "hedgerow", which I think is a bit of a pidgin. That's one way to do it. ))