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.
(..) You want more? Come on. Show your fucking face. You think you're going to get me that easily? When it's tough like this, you know it's gonna be ugly. Better make it quick or else we'll be all day listening to his crap. Asshole! Come on, show me your fucking face, faggot! - Come on, gum trick. (..)1994 Léon: The Professional