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.
(..) And I'm not gonna be able to do that. And I'll just start to resent you for even asking me to stop drumming. And we'll just start to hate each other. And it's gonna get very... it's gonna be ugly. And so... for those reasons, I'd rather just break it off clean. Cuz I wanna be great. And you're not? (..)2014 Whiplash