I hear you -- and I always nod approvingly when I see books of such monsters, but I don't buy them, myself -- but then I think of the black squirrels in the 1E Monster Manual II. If you don't remember them -- and they're not worth remembering -- they're evil squirrels. That's it.To me, they are "mean animals" but also "weird fantasy animals". They are small ways to make your setting slightly different from the real world. It's a classic fantasy trope to have not-quite-horses, not-quite-cats, not-quite-hawks, etc, etc . . .
I haven't used them yet, but I probably wouldn't use them as beasts for the PCs to fight, but descriptive flavor as the PCs travel through the wilderness.
Someone decided to take an off-hand reference in the Hobbit to evil black squirrels in Mirkwood and stat them up, because there was no synonym or version of a monster that did not get stats back then.
But they're just squirrels. They don't get swarm stats (there weren't swarm creatures in 1E). They don't have a special attack. They're just jerk squirrels.
Sometimes, IMO, you can just say "oh, and the wildlife here is weird, man. Deer antlers all appear to be made out of wood, and in the spring, their antlers flower." We don't then need a flowering deer write-up.
In any case, I apologize for the long digression off the topic of the thread. I promise to buy this book to make up for it.