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.
(..) guy you met at the bar the other night. I don't recall meeting anyone who matched that description. I think I'd remember. Oh, all right, you got me. It's the ugly, obnoxious, toothless loser... who got hammered and wouldn't leave you alone all night. Oh, Will! I remember. (..)1997 Good Will Hunting