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.
(..) murderers and rapists... - on the streets. - Murderers? - Murderers, like yourself? - A woman... so ugly inside that she couldn't go on living... if she couldn't be beautiful outside? A drug dealer. A pederast, actually. Let's not forget... the disease-spreading whore. (..)1995 Se7en