"We bite our nails not out of fright but to sap a demon of his might."
Nail-Biting Nelson Nehry is the primordial demon associated with starting the trend of people biting their nails. In days of yore, humankind had the nail clipper but not the sufficient thought power to discard their nail clippings. This led to many bleeding feet, but humankind continued its destructive habits. However, one day, a demon surfaced from the deepest chambers of Hell. Unlike other demon-tachi, its diet was not human, but a human part: the finger nail. And so, the demon terrorized numerous villages with wanton disregard for the inhabitants as it scurried across floors, procuring any finger nail it could.
Eventually, a council was assembled, and the people decided: "NO DEMON WILL EAT MY FINGER NAILS." They proceeded to chomp off their own fingers.
In later times, humankind had evolved to just bite their nails. By then, Nelson Nehry had starved to death.