an 'ugly duckling' that has become a beautiful swan [=an ugly/unsuccessful person or thing that has become beautiful/successful]
('informal') That dog of yours is '(as) ugly as sin'. [=very ugly]
The house is an 'ugly duckling', but it has a lot of potential.
(..) the day after you were born. We were in the hospital room, your mother was asleep... ...and they brought you in and gave you to me. You were this ugly little red thing. And all of a sudden, you grabbed my finger with your whole fist... ...and you squeezed it... ...so tight. And that's when I knew. (..)1995 TvShow: Friends Title: The One with Two Parts: Part 2 Season: 1 Episode 17