From Longman Dictionary of Contemporary Englishugly ducklingugly ducklingUGLYSUCCESSFULsomeone who is less attractive, successful etc than other people, but who becomes beautiful and successful later → ugly
Examples from the Corpus
ugly duckling• The loco is a bit of an ugly duckling.• An ugly duckling, like a printing press, was transformed into a well-behaved goose laying golden eggs.• It's certainly the ugly duckling of the era-until you get to drive one.• But Beryl is an ugly duckling rapidly turning into a beautiful swan ... all through the hard work of recovering mental patients.• However, roses are not swans, and all too often ugly ducklings remain ugly ducklings. 