ugly duckling 
noun
A person who turns out to be beautiful or talented against all expectations.Cosmetic surgery can easily transform an ugly duckling into a beautiful swan, so it is being put to a new use - that is, to create ‘man-made beauty’....
- I was the ugly duckling who emerged a beautiful swan.
- The ugly duckling turned into a beautiful swan, but I'm sadly neither a duck nor a swan.
Origin
From the title of one of Hans Christian Andersen's fairy tales, in which the ‘ugly duckling’ becomes a swan.