Ok, so you want to build a civilisation? Well the first item on the list, is what species to use. That's a major factor in everything that comes afterwards.

Rather than designing the creature first, think about where they're going to live, what they're going to eat.Location is [nearly] everything

A centaur settlement in the middle of the rainforest, isn't going to be practical, at least, not until you've worked their culture out. You can move them later, once you've worked their values out.

To decide what species to use, decide what their native habitat is going to be.

Creatures in mountainous regions need to be sturdy, yet supple, compact, but with power limbs for climbing.

Food is relatively scarce in the mountains, so that limits the size of creatures.

Creatures in hilly, rocky terrain also need to be strong, and well-built, but not to the extremes of mountain dwellers.

The available food is also greatly increased, allowing for far more variety.

Creatures on open plains need to be built for speed, whether to chase others, or to flee. They also tend to be much bigger than others, due to the easy availability of food.

Even in the sea, location matters. On the shallow ocean shelves, offshore, life is wild, and grows abundantly. A hundred miles out, and two miles down, there's very little around, so what there is, tends to be small, and specialised.

You Are What You Eat

…Or at least what you catch. Food is the other major problem, the diet of the creature, greatly affects how it looks.

In other words, until you know what the creature's diet is going to be, you won't know how it appears. You don't have to design a whole ecosystem, of course. Your players may never see the prey your species was designed to catch, but the body-shape they have through trying to catch their prey, affects the rest of their culture, right down the line.
The Workaround

As you can see, the above limitations produce a restricted variety of creatures. How do we increase this? Well, we work around it, by factoring in the terrain, and food supply of the surrounding areas.

If we take a look at this map overlay:

Key: Dark Grey: Mountainous Light Grey: Foothils Dark Green: Forest Light Green: Plain Yellow: Desert Blue: Open Water

This is a map of one small area within a virtual world. Inside one of the mountain squares is a dragon's lair. This location offers a difficult-to-reach nest, that is sheltered from all but the most determined predators, has very little local competition, but demands a creature with physical strength, resilience, and a supple, manoeuvrable body -perfect for the Wyvern that lives there.

For food, however, we look outside the mountain range. There's an open plain nearby, full of four-legged, grazing herbivores. Walking meals for an airborne dragon.

Should our dragon turn out to be a fire-breather, he or she will not want a moist atmosphere in his, or her throat. However, if we look at the map, we can see that the mountain range must trap the moisture, because there's a desert to the east. Therefore, a cave on an eastern slope would be perfect.
Our Creature

For the remainder of the article, we'll work with one particular race. At this point, I have no idea what this race will turn out like, that'll evolve as the process builds it.

So, our creature is a desert-dweller. It came to be, in the middle of a wide, desert-bowl, surrounded by mountains on one side, and sea on the other. The map below shows the rough idea of the area.

As you can see, with the exception of the river flood plain, this area makes for seriously hard living. The floodplain itself, by it's very nature, is barely habitable to a pre-technology creature, built for desert survival. So, we are looking at a thick-skinned, short-haired creature. Fast movement in the desert, except in fleeing spurts, is not recommended, so our creature isn't built for speed.

Given the presence of a water-supply some of the time, our creature needs to be capable of storing large amounts of water, for long periods of time. It is also extremely resilient to heat, and rapid temperature change. The creature in incapable of sweating, instead needing another method of temperature regulation

As for food, our creature is a predator, a long nose for digging up buried creatures, and slim, pointy teeth for quick penetration of tough hides.

Its prey are natively snakes, beetles, small lizards, and some ferret-like creatures that live on the edges of the mountains.

NEXT: Stage 2: Planting the seed