Extinct, Australian animal! I don't think there's much more that needs to be said about how awesome Tazza is.
Or rather, just all of the animals from the Obernewytn chronicles. They were written like normal characters, with personalities and distinct voices, and with intensely loveable traits. They play huge roles in the plot of the series (the series that I'm admittedly only a couple of books into).
Griffin the eagle from Blood Song by Rhiannon Hart
This book convinced me that I need an eagle as a pet. That everyone needs an eagle as a pet. No one's life is complete unless they have a super cool eagle following them everywhere, doing their bidding.
The swan from Flyaway by Lucy Christopher
The story of Isla rescuing the swan was touching, even for non-animal lover like myself. I'm using the descriptor "animal companion" loosely here, but I really loved the relationship between Isla and the swan.
The dog from Only Ever Always by Penni Russon
Only Ever Always was an amazing and complex story, and the relationship between the dog and Claire and Clara was one of the best parts of it. He was one of the key things connecting them, and I liked especially how they weren't always partial to each other.
So what about you guys? Are there any animal companions from books that you fell in love with? And do you have any suggestions for future top 5 list themes?