This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Swan Maiden (Worldwide)
The swan maiden is a mythical creature who shapeshifts from human form to swan form. The key to the transformation is usually a swan skin, or a garment with swan feathers attached. In folktales of this type, the male character spies the maiden, typically by some body of water, then snatches away the feather garment, which prevents her from flying away, forcing her to become his wife.
Both swan maidens and selkies have the common theme of marrying men who steal their skins (feather robes for swans and seal skins for seals). Another commonality is that if she finds her skin she can leave her husband and return to her own kind, and that is what she does in most every tale. Brides who are part animal in fairytales could be rooted in the fact that women have been historically seen as closer to nature.