A yamabiko (山彦) is a Japanese mountain god, spirit and yōkai.
When a sound emitted towards the slopes of mountains and valleys comes back as an echo, it is also called yamabiko when it is supposed to be the response of a yamabiko or a phenomenon caused by a yamabiko.
In this case it is also written 幽谷響. Similarly, it is called kodama when it is the voice of a tree spirit, a kodama (木霊 or 木魂).
Yamabiko Description by region
In Tottori in Tottori Prefecture, a yobuko (呼子) or yobukodori (呼子鳥) who lives in a mountain is supposed to restore the voice of a yamabiko.
In Kusuyama in Hashikami village of Hata district in Kōchi prefecture (now Sukumo in Kōchi prefecture), regardless of whether it is day or night, when a terrible voice is suddenly heard deep in the mountains, this strange phenomenon is called yamahiko.
It is sometimes considered to be the same as the yamawaro discussed in western Japan, as well as the yamako in the Wakan Sansai Zue, and since tree spirits are thought to cause the appearance of the yamabiko, they are also considered to be similar to the yōkai Hōkō that lives in trees.
In collections of representations of yōkai such as the Hyakkai zukan and the Gazu Hyakki Yagyō, the dog-like yamabiko is thought to be modeled after the yamako or penghou. The yamabiko is also thought to be a tree spirit.
The aforementioned yobukodori of Tottori passes for taking on the appearance of a bird, and incidentally, there is also a small mountain in the Kitaazumi district of Nagano prefecture called the "yamabiko rock" (山彦岩) that turns people's voices around, among other appearances of the word yamabiko, and so it is clear that yamabiko as a yōkai is not uniform, both in its origin and in the kind of yōkai it is.