Every K-Pop group has backup dancers and almost every idol became a backup dancer when they were trainees. But here are only a few male idols who were backup dancers when they were trainees.
BTS

BTS is now very popular world wide but did you know some of them were backup dancers? Before their debut Jungkook and J-Hope were backup dancers for South Korean singer Jo Kwon. Jungkook and J-Hope were also backup dancers for Glam along with Jimin and Suga.
Kang Daniel

Before debuting as a member of Wanna One, he was a backup dancer for Cao Lu. Cao Lu is a South Korean singer from the girl group FIESTAR. He is seen at the back, wearing a black shirt.
MONSTA X’s Shownu

Shownu (kpoplove.net/shownu-monsta-x) was a backup dancer for Lee Hyori. She is a singer, producer, activist, actress, and television presenter. He can be seen in Lee Hyori’s performance of ‘Bad Girls’.
Stray Kids’ Lee Know

Predebut Lee Know was actually one of BTS’ backup dancers. Just like BTS, he was also a backup dancer before becoming an idol. Here are some clips of him as a backup danccer of BTS.
GOT7’s Yugyeom
GOT7’s Yugyeom was once a backup dancer for Sunmi. He was 17 when he performed with Sunmi in Hong Kong. Here’s the video, he can be seen wearing a white shirt.
2PM’s Wooyoung
2PM’s Wooyoung wasn’t actually a backup dancer, but a replacement. He replaced Big Bang’s T.O.P’s because the idol was sick. So he had to perform as T.O.P.