Yoshikazu Mera (米良 美一, Mera Yoshikazu) born May 21, 1971, in Miyazaki, Japan, is a Japanese countertenor. His range is three and a half octaves. Mera now primarily sings classical music. He appears frequently a soloist with the Bach Collegium Japan, where he led by Masaaki Suzuki performs Baroque music. It was revealed in a television documentary entitled “米良美一の實話” (“Yoshikazu Mela’s true story”) that he was born with congenital osteogenesis imperfecta. (Source)
Kitsune's Thoughts 
OMG! I always thought it was a woman singing!!!
Yeah, I also thought the same thing! I watched Mononoke at a time when I didn’t care that much about the staff and cast.
If you look on Youtube within the Mononoke Hime voice recording session videos there is also one about Mera doing this song. It’s quite interesting… 🙂
Thank you for mentioning it 🙂