Sometimes you build a world and you want to create everything from the ground up. Other times, you want to take something familiar, and make it strange - tune it to the needs of your story. The first option takes a lot of time and research and thought. The second one might seem easier, but it can be deceptively tricky.
One of the traps that you can fall into is where you take everything familiar in the system and simply give it a new name. Maybe you can get away with one or two things like this, but the more "maros" and "quasits" you have that behave just like deer and rabbits, the more readers are going to look around and go, "Wait, nothing is really new here." If it's a rabbit, call it a rabbit, in other words. And I do - when something behaves like a rabbit, and has the physical, ecological, and symbolic properties of rabbits, I call it a rabbit. (This actually happens in For Love, For Power.)
Are people going to get mad if they are in a world where they expect things to be different, and they find a rabbit? Some are. Really it depends on context. When I used the word grouse in "Cold Words," one reader got mad at me, saying it was stupid of me to put an Earth bird on an alien world. To me, though, it was just a translation. After all, my character appears to be thinking in English when we all know he can't possibly be. Almost every word he says is therefore a translation - grouse was just one more translation amidst all the others, from my point of view. If I'd used a word in the alien language, I would have had to do a lot of extra work to convey the idea that I meant a chubby-looking bird, in a place where it didn't deserve that much attention.
Varin is a bit of an odd situation, because it's designed to be a world where things seem very familiar, and yet are deceptively different. Therefore it's not a problem in my view to include rabbits but also to include tunnel-hounds, which are a bit like eyeless black puppies with a keen sense of smell and platypus-like electricity sensors.
So far, so good, as long as we don't actually go outside into the forest.
Of course, we do have to, though. You can't put a big bad wilderness out there and never go into it! (At least, in my opinion.)
At the moment I am writing a story that requires me to face the question of the wilderness head-on. I've written scenes up there before, but never really done it justice. In particular, there is one uniquely Varin addition to the ecosystem that will make a huge difference to it. It's a tree-that-is-not-a-tree, called a shinca. These are things that look like trees, which have their roots down in hot rock, and grow up through the caverns to the surface, where they branch and have fruit. They are indestructible by ordinary means, they glow, and they are very warm to the touch. They also have offspring in the form of little floating sparks which collect excess energy from the forest, up to and including fires, and bring it back to feed the parent tree. All fun stuff - but the issue then becomes that these trees have grown up with this forest, evolved with it. That means that the forest will have particular properties that are going to be different from an ordinary forest that does not have shinca in it.
To me, that means I have to find an ecological logic for why the forest is the way it is, and how it interacts with the shinca. Each of the shinca's properties will have a different effect.
1. The shinca's offspring prevent forest fires.
This is going to create a very different kind of environment in terms of tree life and underbrush. Trees will not produce hard seeds that need fire to break them open. Underbrush will grow very thickly without creating fire danger. Creatures that eat the thick underbrush will thrive. Fewer trees will have fireproof bark, though they will still have insect-proof bark.
2. The shinca's light will be used as guidance and for safety.
Some insects will navigate using shinca rather than moonlight. There will be some nocturnal animals with less powerful night vision, who stay close to the shinca because they can see more effectively there...and they may be insect eaters! These in turn will attract predators, and will likely have to be very good at hiding because they can't stray far from their home shinca.
3. The shinca's heat will attract wildlife.
The heat will be a helpful side effect for insects and for certain small animals. Other animals may seek the shinca out in case of particularly cold weather. In the autumn, animals who migrate southward to seek warmer weather will use the shinca as gathering points from which to begin their migration. This includes both butterflies and birds, and these congregations of animals will similarly attract predators who are keen to fatten up for wintertime. During winter, the shinca become oases where liquid water and plant life can be found in spite of the surrounding cold.
I'm really excited to see how these differences play out when I'm able to take my story "upstairs" from Varin's cavern cities and really get my characters deep into this forest, interacting with it and its flora and fauna. And I encourage you to think past the surface to the underlying ecology of the forests and other environments you work with. You might discover some really wonderful opportunities.