a dark horse bobbing up的用法和样例：
- produce an unexpected winner; a dark horse bobbing up
- Anna's such a dark horse--I had no idea she'd published a novel.
- Jack is a dark horse; he never says much and you don't know what he's thinking.
- Gerald would be a dark horse to the end.
- He's a bit of a dark horse: he was earning a fortune, but nobody knew.