To be exact, Kokushibo's name was originally Michikatsu Tsugikuni, Yorriichi's twin brother. According to manga, Michikatsu is very strong (not as strong as Yoriichi) and became the Moon Hashira (500 years ago). However, he was very jealous of Yoriichi (cuz Kokushibo/Michikatsu is not stronger than Yoriichi), So he came so far than he asked Muzan to change him into a demon (to become even more stronger so he can defeat Yoriichi). In the end, Kokushibo is not stronger than Yoriichi.
Also, everyone in Demon Slayer is born as humans, but some would be turned to demons by Muzan (demon king). [If u are not sure, you could read manga (infinty castle arc) or search google. :)