Okay, time to learn ZBrush.  You should have ZBrush 4 (its a freebie upgrade).

We are going to use the animal called Shadowbox.

That is located in your light box (taking up space at the bottom of the canvas).
Double click on shadowbox and wait a couple of seconds for it to appear on the canvas.
 
You will need a straight heads on back shot, to do that move the cursor outside of the shadow box and move it while having the shift key depressed. Once it is aligned, release the mouse button first then the shift key.

You will also need to click on transp, and ghost on the right shelf right next to the canvas.
Reason:  you don't want the solid part showing just yet- You want a solid shadow and you can't see through that darn part. 

Then adjust your brush size down;  after that, hold down the Ctrl key and with the mouse or tablet pencil draw in the shape of that wabbit, fill it all the way in.  Then deactivate your transp and ghost to check it, if it needs touching up, hold down the Cntrl and Alt key while erasing with the mouse or pencil if necessary.

Then to make the shadow box go away, go to the subtool menu on the right side of the canvas and deactivated shadow box.

Now you got a purdy wabbit that you mayl need to plane down some.  Try the planer.  If its too much, depress Cntrl Z to return to previous state and on the top shelf, reduce the z intensity and try again.

You now have your purdy wabbit.  Try it!  Jan