Munak/Bongun/YaoJun are base on the "jiangshi" a Chinese zombie. Erroneously translate as Chinese vampire. The costume for all 3types are base on Court Costume for the Qing Dynasty.
The Gangshi Trio is chinese based. A better question is why are Munak and Bongun in Payon. As why isn't Bongun there, could be because he traveled down to Payon and died there, dunno.
Even though its origin is Chinese, it appears in Japanese and Korean lore as well (as "Kyonshii" and "Gang Shi" respectively).