I decided to hold off of Zbrush for a little bit. Instead, I downloaded Pixologic's free sculpting software called Sculptris. From what I understand, the tools are basic tools are the same, but the navigation is a little easier, making it ideal for speed sculpting and as an introductory software. I found it very comfortable to use after making a little practice sculpt:
The auto-symmetry tool is especially wonderful. It cuts the sculpting time to less than half what it would be if I were to sculpt physically with clay or Sculpey. Unfortunately, there is a small limitation to the symmetry tool. It can't be applied after creating an asymmetrical detail to your sculpture. For now it won't be too tricky with all my creatures being symmetrical, unless I wanted to give one a really cool hairdo.
Here is the finished sculpt based off the first in my set of creature designs. I stayed as faithful as I could to the 2D design.
One thing I need to figure out is if Sculptris can render high-resolution images. I'm still working out the small details, but I can see the potential that the program has not just for creature and character design, but for illustration too.