tree spirit
谺 centers on a tree spirit, a supernatural entity dwelling in trees.
The exact historical development of 谺 is uncertain. It combines 谷 (valley) and 牙 (fang), but the connection to 'tree spirit' is not clearly documented.
Picture a valley (谷) where a fanged (牙) tree spirit dwells, guarding the ancient woods.
For コ, imagine a tree spirit co-existing with the forest: co -> コ, and the spirit watches over the trees.