Zenki 前鬼 and Goki 後鬼 were a married couple of demons (ONI)
they were the servants of En-no-gyōja (father of Shugendō)
 

Representation of the yang in the dualism of the taoism (yin/yang), Yang is male, and stay at the front…
Like the komainu, they express with their mouth the A-gyo and Un-gyo (sort of Alpha and Omega, begining and the end)
Zenki carries an axe. The axe rips through the world of the evil…
Their 5 children represents the 5 elements (which can be also linked to the Onmyodo).