We don't have synonyms for ugly duckling yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are ugly ducklings.
(n) :
a young person who is ugly, but who is expected to become beautiful as they mature - from the folk/fairy tale of a duckling considered ugly, but which in fact matures into a magnificent swan, far more beautiful than the ducks
(n) :
a young person who is ugly, but who is expected to become beautiful as they mature - from the folk/fairy tale of a duckling considered ugly, but which in fact matures into a magnificent swan, far more beautiful than the ducks