From A Wiki of Ice and Fire
|Book||The Mystery Knight (mentioned)|
Ferret was a child from King's Landing during the reign of King Daeron II Targaryen. He was both a beggar and a thief.
Ferret was friends with Rafe, Pudding, and Dunk, who would later become known as Ser Duncan the Tall. Once, Ferret stole a head off a spike upon a wall once, when Rafe and Pudding told him he would not dare to do so. When the guards appeared, Ferret threw the head to Dunk. Ferret and his friends escaped the guards with the head, and used it to terrorize the girls of Flea Bottom, by chasing them down and letting them go only after kissing the head. When the flesh on the head turned black, Ferret and his friends threw it into the kettle of a pot shop.
—thoughts of Ser Duncan the Tall