The tengu changed character over time, which is the most interesting thing about them. The name is written with characters borrowed from a Chinese "celestial dog" (tiangou), a portent of the sky, but in Japan the being became a creature of the mountains. In medieval Buddhist writing tengu are hostile: arrogant, deluded spirits, often the transformed souls of prideful or corrupt monks, who obstruct the Dharma and lead practitioners astray, a whole category of tengu-do, a fallen mode of rebirth for the spiritually vain. The early form was bird-like, with a beak and wings; over time the most familiar type acquired a red face and a long nose (the daitengu), while the karasu-tengu kept the crow's head.
Their mountain cult attached itself to specific, named peaks across Japan, of which Mount Kurama, north of Kyoto, is the best known, but Mount Atago near Kyoto and Mount Hiko in Kyushu carried comparable tengu associations of their own, each with its own local legends and its own resident tengu chieftain in popular telling. Mysterious sounds and disturbances in the mountains, falling trees with no discoverable cause, distant laughter, objects moved overnight, were widely attributed in premodern rural Japan to tengu mischief under the general folk category tengu-daoshi, "tengu felling," a documented belief distinct from, though related to, the more elaborate literary tengu of Buddhist and warrior legend.