Watership Down by Richard Adams deserves a mention I think. It's not exactly a series, but it did get a sequel. Richard Adams also wrote The Plague Dogs and Shardik. Shardik doesn't actually take the POV of the animals, but Lord Shardik is a mythical bear that is kind of a main character. I haven't read The Plague Dogs yet, and only learned about it while checking my spellings for this comment, but everything else.by Adams that I've read has been amazing.