AKA Flora and fauna.
The population of a novel’s world is dependent on its life. Animals and plants provide food, shelter and trading opportunities. There is huge, huge flexibility on this, but the one key rule is that it has to make sense: ensure both flora and fauna correlate to the environment.
For example, STXI includes this giant red spider thing on a planet of ice. Bright red: no camouflage. Tiny sharp pincer legs: would go straight through ice and snow. Reptilian skin/no fur (cold-blooded): would freeze. Completely unfeasible for this kind of environment. (Apparently, this planet was originally a desert planet, which would explain a lot of this thing’s design.) But it’s an alien planet is not a good excuse, fyi.
Animals and plants adapt to their environment, driven by changes in their food supply. Stripes develop as camouflage, bright colours as a warning against predators, talons to climb trees, etc. All of these developments are generally weird and completely mind-blowing (the whole web thing, for example), but they make sense. Swamps will have water-based creatures and plants. Deserts will have very few life forms, ones highly adapted to the heat and lack of humidity.
Each terrain will have its own ecosystem and food chain. Apex predators need meat and herbivores require grazing ground. And some of them should feed the population (humans, probably, or the next relevant species). But waste nothing. Consider fur, skin for leather, hooves (glue, ick), hair, teeth – etc. – and how everything can be used.
Traditionally, large herbivores have been used for travel and farming. The longer an army can travel, the further its reach, thus the larger its empire. Horses weren’t introduced in Ancient Egypt until the Second Intermediate Period. Chariots became its greatest weapon.
Use inspiration from the real world in designing these creatures, but think outside the box. The creatures in Avatar were influenced by race cars. Makes for sleek monsters. Description of these will make them come to life, but limit yourself to two or three key elements. (This rule applies to all kinds of description when writing a novel: less is more.) Focus on what is relevant to the character (i.e. if she’s being chased, she’ll be more worried about the size of its teeth and its speed than whether it has stripes or spots. Unless the spots are dripping venom). If they’re being eaten, it’s important to have at least some knowledge about its physiology to tell that the succulent liver is beneath a hide three inches thick just under the ribs (or something).
And don’t forget to write this stuff down. Continuity in worldbuilding is crazy important. Don’t give that ice monster fur at the beginning then decide to shave it off half way through the second Act.
Flora contribute to the ecosystem as well as the environment (oxygen). As with fauna, each plant has its own purpose (still can’t figure out what the cockroach is for…). Many of them will be used for food (agriculture) and will bloom at different times of the year. Seasonal fruits are often a delicacy because they are ripe for a limited time and usually only in the right climate.
Different trees have different qualities. Willow bark was used as a primitive pain killer, and oak trees make great furniture. Wood will burn at different rates and some will make better, more waterproof shelters than others. While writing, the most important question to ask is why. Why this tree for that shelter, or that plant for this seasoning. The more logical a world, the more real it will seem. (Not to discount a little bit of chaos, now and again.)
Finally; trade. Some of the most precious items in history have been luxurious furs, exotic plants (or drugs from those plants) and animal bone. The clan or city with access to those items reaps wealth. Rare = expensive. They are also a status symbol (more on clothes later). People will pay good money for a purple mink coat if other people can see them wearing it.
Pandora is a decent example of well-designed animals and plants. Do you have any others?