It's fitting that Harry and his buds start taking Care of Magical Creatures this school year, since we have a lot of animals running around in this book. There's various Animagi (Animaguses?), as in wizards and witches who can transform into animals; there are Patronuses (Patroni? someone needs to publish a Harry Potter grammar guide) that resemble animals; there's Harry's ever-faithful and smart owl Hedwig; there's Buckbeak, the hippogriff who's hugely important to the plot; there's a werewolf, who is also Professor Lupin; and there's Crookshanks the cat, who pretty much steals the show in some places. So why does this novel resemble a bizarre menagerie?
Well, it's notable that all the animals featured in this book are smarter than your average bear, er, animal. Many of them are people in disguise and the ones that aren't have very strong personalities and are clearly quite intelligent (see genius Hermione's super genius cat).
In a way, this book is largely about respecting animals and taking them seriously. Crookshanks is far from just a stupid, mean cat, as Ron often yells. Buckbeak isn't just a mindless brute – he attacked Malfoy because the kid didn't show him proper respect (6.2.61-68). And Scabbers is far from a boring, scabby old rat. While we could read this as animal rights advocacy, we think there's something else going on here.
Animals represent themes of appearances being deceiving. From the grim that turned out to be Sirius, to the childhood pet that turned out to be a traitor, animals constantly stand in for disrupted expectations. Through the various animals in the book, Harry and his friends learn to look beneath the surface of things, and to not just dismiss things just because of how they first seem, be it stupid or dangerous or boring. There are hidden depths to everything, not just animals, in this novel.
Animals also reflect certain things about the people around them, sometimes unintentionally. It's extremely fitting that the traitor Pettigrew turns into a rat, after all. And Crookshanks acts like a cat version of Hermione – brilliant, relentless, and too curious for her own good sometimes.