"Some shape-shifters are able to change form only if they have some item, usually an article of clothing. Most of these are innocuous creatures — even if they are werewolves. In Bisclaveret by Marie de France, a werewolf cannot regain human form without his clothing, but in wolf form does no harm to anyone.
Another such creature is the selkie, which needs its sealskin to regain its form. In The Great Silkie of Sule Skerry the (male) selkie seduces a human woman but does no further harm.
The commonest use of this motif, however, is in tales where a man steals the article and forces the shape-shifter, trapped in human form, to become his bride. This lasts until she discovers where he has hidden the article, and she can flee. Selkies feature in these tales. Others include swan maidens and the Japanese Tennin.
Various forms of fairytale fantasy have taken up these creatures and incorporated them into modern day works. Jane Yolen took up the notion of selkie in Greyling and transformed it into a foundling tale..."