This is one of my favorite aspects of fantasy, science fiction and horror. I watch nature documentaries incessantly and I approach creature creation from that view point. In Court of Scales, my dragons have three sets of wings, breath that smells like an alchemist’s workshop and scales that grow cloudy-white with age.
The more important aspects, to me, are that they’re solitary creatures slowly becoming social ones. It’s a long process, taking several hundred years already and still not very advanced. They have a voluntary political system because there’s no way to force them and only the more social dragons even bother to gain position. They mate for about a month every two years and the mothers raise their offspring without any contact with the fathers. While I find all that very fun to play with, what’s even more fun is to look at the exceptions. Razier, the main dragon character, is one of those and desperately wishes he wasn’t. Poor thing.
I really enjoy looking into the instincts of a creature and pondering their evolution. In another story, I’m playing with the concept of the dwarf. Dwarves, as they are commonly thought of, probably didn’t evolve underground, but rather moved there sometime in their past. Dwarves that evolved underground have no reason to have eyes. There’s no light. There’s no way to see, so why would nature give them organs to do it? What would a story written from the “point of view” of a creature without eyes be like? How would they communicate? How would their society have come about and evolved? What would it mean to them? How would they sense, get around and interact with the world?
I find these questions fascinating and I like working on the story because it forces me to come up with new ways of describing things, new ways of understanding things. There’s even—currently—a dream scene description that I adore. It may get cut, but right now it makes me smile every time I read it.
Creatures don’t have to be sentient, of course, although I think it makes them more interesting. I like creating the non-sentient creatures, too. Birds that attract mates by singing a particular note, lizards that eat sewer rats and small foxes on the canals of my story’s setting, dogs the size of horses that pull sleds like elaborate carriages, and birds with pine-tree-branch wings. Then there’s the weird stuff, the burrowing rock mites, sand-eating lizard-birds and spacefaring cephalopods.
My philosophy of creature creation is to start with a detail and ask lots of questions. When creating creatures, sometimes I start with a sketch. I have a notebook that I call my bestiary and I sketch out any ideas I have. More often, though, I start with a non-visual detail: an interesting system of magic, an aspect of their physiology, an idea of how they find nutrition. Then I work backwards.
Generally, life needs water and nutrition. To survive more than a generation it needs to be able to reproduce. These are all basic, but once you get an idea of the basics, one thing leads to another. For instance, if you start with the idea of a plant-based sentient life form, you know that your creature’s going to need light, possibly other nutrients, water and a means of reproduction. Plants have a whole range of interesting reproduction practices, and some only produce flowers, fruits and seeds when under stress. That could be pretty fun to play with, a character that starts to flower when under stress. How long does it take? What resources (light, water, nutrients) does it use? Does it have other effects on the creature? Do they start flowering immediately? Do some types of stress have a greater effect than others?
I think questions are the best way to create a creature. Answering the question creates other questions to ask and answer. The answers build the creature up from that one idea. Well, that's my system at least. :D