The Ugly Duckling is a children's story written by Hans Christian Andersen and first published in 1843, to great success.
The story tells us the story of a duckling that is especially bigger, clumsier and uglier than his other brothers, who messed with him for it. However, with the passage of time he realizes when looking at a reflection of the water that he has become a splendid and beautiful swan.
It has been adapted to other media on many occasions, both to the cinema, opera or musicals. Unlike other Andersen stories, which were collected from popular folklore, this one is totally original and written by the author as an analogy with the stages of his life.
The expression being "the ugly duckling" has settled in Spanish and is applied when a person who is rejected at first impression, later becomes something much better and is better seen.