An ugly duckling is a person who at first seems unpromising but later becomes admired . It is based on the title of a tale by christian Andersen.The following piece of information has been copied from Wikipedia:
The story tells of a homely little bird born in a barnyard who suffers abuse from his neighbors until, much to his delight (and to the surprise of others), he matures into a graceful swan, the most beautiful bird of all. The story is beloved around the world as a tale about personal transformation for the better.“The Ugly Duckling” was first published 11 November 1843 with three other tales by Andersen in Copenhagen, Denmark to great critical acclaim. The tale has been adapted to various media including opera, musical, and animated film. The tale is completely Andersen's invention and owes no debt to fairy or folk lore.