By my definition, then, you can’t have a true animal first-person narrative in fiction; only a third-person with insights into the animal’s thinking, who explains him for the reader. A first-person turns into an anthropomorphised human whose concerns have been altered due to his form. We can’t know an animal that intimately without entering an alien world of urges and primal drives. Since that would be alien to us, we must make the animal articulate, which is to make him human.
For example: Being Lee's horse, he loved the General, so at Appomattox, he believed Lee was there to receive the Yankees' surrender. That kind of limited awareness made the book wonderful in my opinion.