Is Biter from Beyond the Wall?
In the Prologue to A Dance with Dragons, Varamyr muses about the fate of the various subgroups of Free Folk, listing off "[the] Thenns, giants, and the Hornfoot men, the cave-dwellers with their filed teeth, and the men of the western shore with their chariots of bone . . ."
Could it be that Biter came from these cave-dwellers and that is how his teeth came to be filed?