Kodama

Here's my take on a Kodama (or Kiinushii on Okinawa Island), a forest yokai whose name literally means "tree soul/spirit". Each Kodama is tied to a specific tree, but only certain trees have one ; by the way, these trees possessing a spirit are also referred to as Kodamas, but I won't do that for the sake of clarity.

It is generally impossible for the layman to guess whether such or such a tree is inhabited by a Kodama, unless he has been informed by the elders of the region, but often, the older and taller the tree, the more it's likely to be endowed with such a spirit. But this yokai seems to manifest itself very rarely outside of its vegetal home, and is considered polymorphic, being able to adopt the form of a will-o'-the-wisp, an animal, or even a human (which allows certain Kodamas to date humans they would have fallen in love with more easily) ; therefore, it may be difficult to identify them.
However, anyone cutting down a tree inhabited by a Kodama would bring a curse on him, and even on his entire village. Thus, custom dictates that a shimenawa (sacred rope made of rice straw from which hang strips of folded paper) is tied around any tree sheltering one of these yokai, to signal its presence and preserve the Kodama as well as its human neighbors.

In some regions of the Japanese archipelago, these spirits are the subject of more elaborate veneration. Indeed, on the island of Aoga-shima, shrines are built at the foot of the largest cryptomers/cedars of Japan growing in the mountains, in honor of Kodama-sama. On Hachijo-jima, after the felling of a tree, a ceremony aimed at appeasing the spirit of the plant must be systematically held. On the island of Okinawa, it is good form to pray before felling a tree, and it is said that if the echo of a falling tree is heard in the middle of the night when no tree has really been cut, it means that a Kiinushii is dying, and that its tree will wither a few days later.
Kiinushiis are sometimes confused with Kijimunas, other small tree-dwelling yokai endemic to Okinawa. Also, if the echo phenomenon in the mountains is most often associated with the Yobuko/Yamabiko, the Kodamas are sometimes blamed for it.