In order to get the lighting right on creatures that don't actually exist, I find it's easier and faster (for me) to sculpt a model out of clay first. That way, I can light the scene in any number of ways and make it as realistic as possible. I still have to figure out the colors, but at least I have a guide. These Sculpey figures are now standing on a file cabinet in my studio. I'm running out of room for them.
Oh, and the human figures in the above painting are both me. But I won't post the photo reference for that part of the painting, as it's kind of embarrassing.