Books with animal protagonists often blend real biology with imaginative twists. The border between 'based on' and 'purely fictional' gets wonderfully fuzzy. Take Richard Adams' 'Watership Down'—those rabbits behave with a heightened, almost epic consciousness, yet their social structures, dangers, and instincts are deeply rooted in observed rabbit behavior. It's not that they have laser eyes or wings; their 'unlikeliness' comes from the elaborate language, mythology, and governance they're given. That feels more like an intensification of reality than a departure from it.
On the other hand, some creatures are clearly invented from whole cloth but borrow a familiar shape to help us relate. I'm thinking of the daemons in Philip Pullman's 'His Dark Materials'. A talking pine marten or insect isn't unbelievable in a fantasy context, but their fundamental nature as externalized souls tied to humans places them squarely in the fictional realm. They use the 'skin' of real animals to explore a completely fantastical concept. Pullman isn't trying to write a zoological study; he's using animal forms as symbolic vessels.
Then there's the middle ground, like the spiders in 'Charlotte's Web'. A spider that weaves words is, biologically, impossible. Yet E.B. White's genius was in grounding Charlotte so thoroughly in the real lifecycle of a barn spider—her egg sac, her method of catching flies, her natural lifespan. The single magical element feels earned because everything else is so authentically observed. The 'unlikely' animal here serves a specific narrative and emotional purpose without requiring a whole new bestiary. It's a light touch of fantasy on a very real canvas.
Ultimately, I find most memorable animal characters are hybrids. They start with a keen observation of real creature traits—the cunning of a fox, the loyalty of a dog, the solitude of a cat—and then the author projects a human-like interiority or a magical function onto that stable, recognizable base. That combination is what makes them stick, because we can both believe in their animal nature and connect to their personified struggles. Pure fiction can feel arbitrary, but a grounded exaggeration often resonates deeper.