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.
(..) So... With all that, I didn't give you my little present. It arrived this morning, but... I didn't want to ruin the party, in case she was ugly or something. You never know. But at least she replied. Well... Good night! Alright, I'll open it because knowing you, it's gonna take ages... (..)2011 Intouchables
(..) How long has it been going on? He's such a pain in the ass. Six months. Six months? And you've never seen her? She really might be ugly. Or fat. Or even disabled. You should add that after the poem: "By the way, how much do you weigh?" Write it. (..)2011 Intouchables