One way to come up with a whole bunch of unique creatures that also helps build your world’s immersion and believe-ability is to do what nature does:
Start with the environment first, and then adapt the wildlife to fit.
This works really well when you start off with an environment that is, itself, somewhat unique or dangerous to everything living there, while not being an absolute death sentence. The ideal being just enough to put anyone travelling through on edge or grab their attention.
Let’s take, for example, one of my favorite settings (even if I could go on a long rant about the story it got), Zendikar. For those not in the know, Zendikar is a setting (more specifically, a plane of existence) filled with vibrant and lush wildlife and impossible gravity-defying geology. Rocks hanging in midair are par for the course. But the key facet about Zendikar is the “weather” phenomena known as the Roil.
How do I sum up the Roil?
Imagine if the land beneath your feet and the vegetation that grew there changed as easily and swiftly as the weather in the sky.
This makes the land interesting, dangerous, constantly new, and keeps it from being subsumed in local cities or townships. Ruins dot the landscape, being revealed and hidden away like wrecks in the surf.
Forests wash over what was once mountains, and the next day it could be a lake. Different areas are affected more or less frequently, but there is no escaping it. Mapmakers have never had such good job security. EVERY horizon could be a new one, even those you just walked over last month.
So, here comes the question: How does the wildlife adapt?
Well, there are a couple ways to adapt to any threat. Avoid it, beat it, or become immune to it.
The birds of Zendikar take advantage of the floating rocks and debris about the plane as semi-stable nesting sites, and some can live their whole lives never touching the ground once.
The creatures known as Baloths (think a combination of a bear, a bull elephant, and a Barbarian with unlimited Rage capabilities) grew tough enough to withstand the voilent changes themselves, while being strong enough to leap through the trees and take down prey. Their diet grew wide enough so they can effectively act as a sort of predatory scavenger, eating just about anything they can catch, which is most things.
The trees on Zendikar are impressive sights, growing huge twisting trunks that interweave or spiral on their own, effectively being able to overgrow just about anything without having to worry about the time constraints of doing so. (Which is why so many fast-growing trees on Earth are hollow or otherwise light, so they don’t have to spend more time/energy doing things that aren’t filling up as much open canopy as possible before the other plants get to it)
There are other general ways to adapt to threats, which I won’t go into here, but by and large most of the wildlife on Zendikar has been required, simply by where they live, to be either able to adapt and thrive in violently-changing settings, or to be able to avoid or ignore said violent changes in the first place.
So, that in mind, what sorts of general risks or threats could your own settings have? How would the birds, beasts, the herbivores and the insects, the plants and the fungi, all survive those threats? How could they adapt, from the general concepts we have for them now, to be better suited to live out their normal lives in that environment?
Maybe they’re good with camouflage, maybe some have thick hides/fur/armor/scales/feathers/etc, maybe others simply know when and where to not be around or have some protective home they create. Maybe they take advantage of another species’ defenses against the threats.
Maybe they can even turn the disadvantages of their environment to their favor!
The setting your wildlife lives in can be just as useful a tool as any in your belt. Don’t be afraid to cut loose with it sometimes.
Oh, and for those interested, you can even get the info needed to run the Zendikar setting for D&D 5E. Try it, and let me know what you think!