This is a statue of Zao gongen, a uniquely Japanese deity created under the influence of Shinto-Buddhist syncretism. Representative of faith of mountain ascetics, this image was obtained in a miraculous reception by En no Gyoja, the putative founder of Shugendo (Way of Practice) of mountain ascetics at Kinpusen-ji Temple in Yoshino. Amidst it frenetic energy, one senses the calm and regular features characteristic of late-Heian period sculpture.  
Audio guide 

The statue of Zaō gongen was created exclusive Japanese deity under the Shintō and Buddhist syncretism. It is believed to visualize the image that the founder of Shugendō (the way of practice and mystical powers), En no Gyōja had inspired. The lost right arm might have been raised upward to hold a Vajra pestle.
 
Shigeki Iwata 
Masterworks from the Nara Buddhist Sculpture Hall at the Nara National Museum. Nara National Museum, 2010, p.139, no.145.