There's a 3D graphing application for TI-83+ series calculators [1] and an easter egg was that it could plot the head of a duck. [2] (it had the equations for the surfaces in the code somewhere, presumably.)
The first thing that came to my mind when I saw that you can upload an equation on this site is, 'I wonder what the equation is for a duck?'
I don't know. However I would guess it's because the main portion is a sphere and that's easy enough to show on the 96x64 pixel screen of the 83+ series calculators.
