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.
(..) Dialogue: Marked=..,Default,NTP,!Effect,He doesn't understand\Nwhat it's like, lying there day after day... Dialogue: Marked=..,Default,NTP,!Effect,...watching people press\Ntheir ugly faces in on you. Dialogue: Marked=..,Default,NTP,!Effect,Can you hear me? (..)2001 Harry Potter and the Sorcerer's Stone