This is in Child, volume 5, page 203 of the Dover edition. He says it "has not unnaturally been taken for a relic of a traditional Scottish ballad... It is, in factm a stanza from the Danish ballad 'Moderen under Mulde'. translated by Jamieson, and given in the notes to the fourth canto of Scott's Lady of the Lake." So it is a traditional ballad, only not in English.