Procedural Materials
Materials created using procedural blender nodes. That gives unlimited possibilities in resizing without any loss of quality.

Iris Colors
40 color presets and an infinite amount that you can create.Iris Colors Instructions 

Pupil Shapekeys
12 shapekeys that provide opportunities to create a wide variety forms of the Pupil.Pupil Shapekeys Instructions 

What you get
Blender file in which two models with materials: sclera and iris-pupil.
Write your desires about extra forms for the Pupil.

Update 1.1 : Smaller Iris Size

Now it is possible to make the iris smaller.

Also in the sclera material, there is now initially a Hue Saturation node for changing the color of blood vessels.

For the reason if your creatures has different color of blood.

Iris Size Instruction

.