Who is Mr. Quackers?

The first written record of Mr. Quackers comes to us from Julius Caesar's Commentarii de Bello Gallico (Commentaries on the Gallic War).

Julius Caesar meets with Diviciacus, a druid, mage, and leader of the Aedui tribe. Diviciacus warns Caesar of the Germanic King Ariovistus, who he claims is an imminent threat to Rome. After an intense three weeks, and a decisive battle, the Romans defeated the Germans and cemented total control over the region. 

During the celebrations, Diviciacus confronted Caesar, pale faced, with fear and bewilderment in his eyes, and whispered: "Congratulations, my king. But remember, you too are mortal. Beware of the true king of kings, Mr. Quackers. His reign shall be forever". Caesar gasped, and ordered Diviciacus to repeat himself. But when he met Diviciacus' gaze,, his face full of color, and the fear and bewilderment was gone. He knew not what he said, and could not recall his vision. Thus was the power of the Druids. And thus is the  power of Mr. Quackers.