Selkie

Selkies are creatures that live in the sea which can shift between the form of a seal and that of a human. There are stories about humans stealing their skin while they're in human form and hiding it, so they can't go back home. They marry the seklies, which are usually female, and they raise their human children and care for the home. They make good wives, as long as they don't find their skins. If they do, they'll kill their human husbands and go back to their families in the sea. They can still be seen greeting their human children on the beach though.