As a kid, I really loved the Guardians of Ga'hoole books! The film is probably slightly more well-known, but the books were very formative for my love of xenofiction and especially owls (my favorite animals <3). I was so obsessed with them that I shared them with my dad, and he even took me the movie when it first came out. Deadass, nothing was ever cooler to me than birds wearing armor and fighting for justice.
The other dark, sort of mature animal stories I read were Watership Down (still a massive fav to this day), Warriors and Animal Farm. Other than those, I occasionally read some of the Goosebumps and Bailey School Kids books.