Hmm, for me, when someone says "animal protagonist", this comes to mind:

Something fantastical, specifically within a fable/Grimm's Fairytale setting. Though, this is definitely based on the art styles I've associated with animal protagonists, the stories I've read, and the stories I want to create. I grew up reading Frog & Toad Are Friends, Richard Scarry's Busytown, and many of the Grimm Brother stories, and the art style was often hatch-shading, loose sketches, and flowing in a way. They were also fantastical, a bit on the darker side, or very slice of life (but less modern).
So whenever I hear "animal protagonist", I think that. Granted, this is really based on what I would like to see more of. I have enjoyed stories like Beastars that took anthropomorphic animals in modern settings. But I also would like to see more of that hatch-shaded, "delicate pen" art-styled fantastical stories like Totsukuni no Shoujo.