This video briefly explain how to use the Multi-Eye Ztool (a simple ZBrush mesh with sculpting layers) can be used to create eyes.
These are a couple of images created entirely with the Multi-eye tool. These images might not show the most realistic eye, but they could be used as a starting point for further refinement.
The polypaint from the layers was turned into a texture map using the ‘Create Texture from Polypaint’ option in ZBrush. The renders were created in Marmoset Toolbag.