its the god. so, because shiro is possessed by a god since he was a baby, he always thought that he is a god ~ so basically shiro and kuro are both possessed.
Shiro became confused when he heard the crows call him "Human" and said he was abandoned on the mountain when he was a baby and raised by the mountain god, but because of humans the mountain was gone.
Shiro's mentality was becoming unstable so i believe he or the power of the true mountain god made him forget everything including who he is because of how unstable he was becoming.
It turns out Shiro is and has always been human, but like they said above, he never realized it because he has always been possessed by the real mountain god, in a similar way to how Kiyoto is possessed by the black dog. The difference is Shiro was abandoned there as a baby and the mountain god probably possessed him in order to keep him alive.
Thank you guys
Huh? Can someone translate the english please??? I didn't understand what happened at the end...