A yamabiko (山彦, 幽谷響) is a spirit/youkai/god known in the Japanese folklore. In modern Japanese, yamabiko (やまびこ, 山彦 ["幽谷響" can't be used here]) is a word that refers to the phenomenon of the "reflection of sounds in mountains or valleys". In the old days of Japan, people thought echoes occurred because a yamabiko living in the mountains replied back or manipulated the sound. The wilds of Japan are full of strange phenomena, like echoes that bounce back with more delay than they should, or that come back slightly different from the original sound. When the false echo comes from the forest, it is usually attributed to a ko-dama. When it comes from the mountains, it is due to these yamabiko. Yamabiko are known almost exclusively by their voices alone, which are skilled at mimicking any sound, including natural sounds, human language, and more recently, trains and cars. They also occasionally unleash terrible and mysterious screams deep in the forests that can carry for very long distances. Little is known about these yokai due to their rarity and elusiveness. They live deep in the mountains and make their homes in camphor trees, living in close proximity to (and sharing a common ancestry with) the other tree and mountain spirits. For many centuries, their calls were speculated to be a kind of rare bird, other kinds of yokai, or even a natural phenomena. It wasn’t until the Edo period when determined yokai researchers like Sawaki Sūshi and Toriyama Sekien were able to confirm the creature’s existence and record its true shape.
Yamabiko in Touhou
Yamabiko were once a common youkai that lived on every mountain. When the rumors within the humans of the village created a natural explanation for the voices of the yamabiko, many humans had stopped shouting at the mountains. This resulted in the death of many yamabiko and their population continues to decline, threatening their species to a natural extinction.
Yamabiko are the only youkai in Gensokyo who are known to be threatened to extinction due to reason of the natural world. Although, because the health of their population was never stated, it is not known to what extent their numbers had dropped to or the rate at which they're continuing to drop.