Here's the illusion as first conceived by Joseph Jastrow that first appeared in Harper's Weekly in :
The delicately balanced ambiguity of the image, the neutral positioning of the eye; it's an ingeniously crafted portrait that appears to be two things at once.
That real life hybrid above, though? That really is two things—a duck bill united in unholy taxidermy with the back of a rabbit head. Creepy... but effective.