Perhaps "The Great Silkie of Sule Skerry," a traditional folk song from Shetland and Orkney. A woman has her child taken away by its father, the great selkie of Sule Skerry which can transform from a seal into a human. The woman is fated to marry a gunner who will harpoon the selkie and their son. (Child ballad number 113)

and this thread
https://mudcat.org/thread.cfm?threadid=39820
Supernatural Ballads....??