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.
(..) What do we got? AID: -year-old had a huge anterior wall MI this morning... collapsed while jogging. Paramedics resuscitated him, but his EKG's pretty ugly. AID: He's hypertensive. MAGGIE: This diagonal looks tight. Who cathed him? (..)1998 City of Angels