who was the second demon muzan created

The second demon Muzan Kibutsuji created is not explicitly known. However, it is known that Tamayo is the second oldest known demon in the story of Demon Slayer. Other candidates like Kokushibo and some random demon are speculated but there is no official confirmation on who exactly was the second demon created by Muzan. The first demon is Muzan himself, and Tamayo is recognized as the second oldest demon known in the series.