Building a series of 3D critters is a time consuming process. A brilliant software package called 3D Studio Max is widely used for such 
3D design. 3D Studio Max is used for commercial special effects, but can also be used for simple 3D objects.
Using the same "bent" cylinder for the body, each creature has it's own special appendages. If you look closely, you'll see they're just simple geometric shapes. Then, metal textures are applied to the shapes (using "bump" and "diffuse" maps) giving as lovely worn effect. 
Lastly, the critters are touched- up in Photoshop. And suddenly they're alive, adding character to all kinds of DDSN presentations.