In Harry Potter, these are simply the Scottish version of Merpeople,1 more generally they figure in Norse, Celtic and Icelandic mythology and use seal skins to shape shift, spending much of their life at sea as a seal.2
Harry Potter Wiki "Selkie" Last Updated: 2021-08-04. Last Viewed: 2021-11-11. ↩
Wikipedia "Selkie" Last Edited: 2021-10-23. Last Viewed: 2021-11-11. ↩