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.
(..) Gout playing up, sire? It's the damp. It's the only possible explanation. Get me a brandy. The mood of the people, sire, it's turning ugly. They're commoners, Alfrid. They've always been ugly. It's not my fault they live in a place... ...that stinks of fish oil and tar. Jobs, shelter, food. (..)2013 The Hobbit: The Desolation of Smaug