dark horse
noun [ C usually singular ] uk/ˌdɑːk ˈhɔːs/ us/ˌdɑːrk ˈhɔːrs/
(SECRET)
UK不爱张扬的人；（尤指）深藏不露的人，内秀的人
a person who keeps their interests and ideas secret, especially someone who has a surprising ability or skill
Anna's such a dark horse - I had no idea she'd published a novel. 安娜可真是深藏不露——我一点儿都不知道她已经出版了一本小说。
(WINNER)
US黑马（在赛马或竞选中出人意料的获胜者）
a horse or a politician who wins a race or competition although no one expected them to