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.
(..) Those are the Plains of Silence. Stay right where you are little Joey. Kinda lost it's novelty out here. You having a baby? Warlord Junior. Gonna be so ugly. It could be a girl. You killed people with that do ya? Killed everyone I met out here. Headshots. All. Snap! (..)2015 Mad Max: Fury Road