개천에서 용난다 -- A dragon comes out from the creek. This expression is used to describe a situation where a magnificent person of great stature comes from a household of little importance or worth. (보잘 것 없는 집안에서 훌륭한 인물이 났을 때 쓰이는 말)

[edit] English expressions

• To rise from a humble family
• A black hen can lay white eggs
• A diamond in the rough
• A rags to riches story.
• An ill cow may have a good calf.