What is "swan maiden"

The Swan Maiden is a mythical creature who shapeshifts from human form to swan form. Despite the name, males are found in a small number of legends. 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 (usually bathing), then snatches away the feather garment (or some other article of clothing), which prevents her from flying away (or swimming away, or renders her helpless in some other manner), forcing her to become his wife.

There are parallels around the world, notably the Völundarkviða and Grimms' Fairy Tales KHM 193 " The Drummer". The parallels do not necessarily feature a swan per se, she may be a selkie or mermaid.