It is said that if a demon can take on the form of a human, then it is truly powerful. True enough, while non-humanoid demons, especially the anatomically ambiguous worm-bodied demons who are enslaved by Naraku, are strong enough to be trouble for normal humans, they pose little threat to spiritual humans such as monks or priestesses. Those that take on human form are far more formidable. Still, even many of these yōkai have "true forms", such as Sesshōmaru whose true form is that of a giant white dog. Others, such as Entei or Ryūkotsusei, either cannot or choose not to take on human forms, retaining an animal-like form at all times. Among plant demons, Kaō seems unique in his ability to take on a human form. Most plant demons are sedentary and cannot change their shape.
Social behavior among yōkai is also extremely varied. Some demons are very social, especially canine demons, such as dog or wolf yōkai, the latter of which travel in large packs and associate in vast tribal networks. Other clan-based demons include the Panther tribe and the moth yōkai, such as the Hyōga clan. Many other demons are solitary hunters who sometimes fight in small groups and roam across the countryside. Especially weak demons sometimes congregate by necessity or fear, especially those who serve Naraku.
Most demons usually only associate with their own kind, unless driven by some pressing need or self-interest, especially when overwhelming numbers are needed; but without a strong leader like Sesshōmaru, Bikuni, Izumo, Hyōga, or even Naraku, they rarely do so on their own. Bikuni is a rare example of a demon, said to be very old, who is able to unite a diverse group of unrelated demon species into a cohesive army that competes for territory with a clan of imps under the rule of Jaken. The Hyōga clan is an even more extreme example, as they possessed large hordes of demon soldiers who served under them. Not only were these demons of various species, but they also wore uniforms and wielded a wide range of weaponry. Some demons who were grateful for Tōga's protection in the past temporarily gathered under the nominal leadership of his son, Sesshōmaru, to combat the Panther Tribe, but proved ineffective at best against the power and discipline of the panther demons, against whom only Sesshōmaru himself seemed effective. However, only Bikuni was able to gather a large group of diverse demons to serve under him and live as a permanent social unit. Hyōga, by contrast, wielded his influence over his servants during times of war, and only his clan of fellow moth demons actually served him on a day-to-day basis.
A few demon clans such as the Wolf Demon Tribe, the Hyōga clan, and Princess Abi's bird clan, have humanoid demons who travel with related demonic animals (i.e. rather than being human in appearance with animal characteristics, these demons are animals that have achieved near-human intelligence and live alongside more powerful demons who share their nature/animal typing). For instance, the Wolf Demon Tribe consists primarily of demons who have wolf-like traits, but many actual wolves live with them and hunt alongside them, with the eldest among them being capable of human speech. While it is unclear if these wolves possess demonic powers, the demon birds who serve Princess Abi and the demon moths under Menōmaru are capable of various (often quite potent) abilities.
In relation to humans, demons either ignore or are outright hostile to humans, though they generally fear reprisal from priests, priestesses, and monks. Others, particularly powerful demons, have little to no fear of humans. A rare few demons are friendly towards humans, and even mate with them, producing half-demons. However, such behavior is considered eccentric by even the most open-minded demons, and is generally frowned upon, with the offspring of such unions being reviled and discriminated against. Some half-demons are also created when a human offers their soul to demons and are reconstituted as an entirely new creature. However, even Naraku, who is such a half-demon, is looked down upon by weaker demons, as they can still sense that he is part human.
The most powerful of demons are known as daiyōkai. These demons are extremely powerful, are virtually immune to normal methods of human attack, and normally have a number of weaker demons who serve under them. Naraku, despite being a half-demon, also attempted to style himself as a daiyōkai, especially after he reconstructed his body at Mount Hakurei, following which his demonic aura was greatly strengthened. Nevertheless, demons could still sense that he was not a full demon. The greatest of daiyōkai, such as Tōga, Ryūkotsusei, the Panther King, and Hyōga, are colossal demons of enormous size and insurmountable power, able to lay waste to entire regions. While most demons keep to themselves, only attacking what they need for resources or food, daiyōkai often have ambitions of power and conquest, leading to wars between them.
For all of their abilities and for daiyōkai, longevity that often stretches into centuries, demons are not immortal and will grow old and die. In the Feudal era many demons in the series are shown being eliminated due to conflicts with humans or with other demons. Due to this, is rather unusual for a demon to pass away from old age.
In the modern era, there are no known yōkai to exist except for the Tatarimokke, or Soul Piper, and even then it is only visible to humans with great spiritual power. Regular yōkai appear to have either become extinct or hidden themselves amongst the far greater numbers of humanity and now only officially exist in myth.
If extinct, it is possible that they either killed each other off or were exterminated by humans with the introduction of more advanced technology. Another possibility is that they assimilated into human populations over the centuries and their bloodlines were completely absorbed.
Even if they are not extinct, it would make sense for them to not have themselves stand out or draw undue attention to themselves; given there exists technology that can wipe out cities now like nuclear weapons, its likely more than enough to kill a demon. As a Feudal Era hanyō growing up in modern times, Towa Higurashi was taught to not use her superhuman abilities in front of humans.
Spirit & Yōkai types
- Oni (鬼, "Ogre"): Oni are hideous giants with sharp claws, long horns, and wild, untamed hair. They are generally humanoid but occasionally they are shown with odd numbers of eyes or extra fingers and toes. Their skin color is usually red, blue, black, pink, or green. Oni typically wear tiger skins and carry fearsome iron clubs called kanabō (金棒). In the earliest legends, oni were benevolent creatures who fought off evil and malevolent spirits.
- Kappa (河童, "River Child"): A water-based demon with webbed hands and feet, a turtle shell, and a circular dish on top of its head. It can exist out of water only as long as the dish contains water. About the size of a small child, the Kappa is very strong. It attacks horses, cattle, and humans, dragging its prey into the water, feeds on their blood, or drains their life force, (etc.) leaving nothing behind except a hollow gourd. It is said to sometimes drag unwary travelers into the water and drown them. In many localities, drowning is still referred to as GAPPADOKO. When benevolent, the Kappa is supposedly a skilled teacher in the art of bone setting and other medical skills. The Kappa is always portrayed as trustworthy despite its many evil ways. When captured and forced to promise never again to harm anyone, the kappa always keeps its promise.
- Kitsune (狐, "Fox"): A fox apparition, and one of the HENGE (Shapeshifters). The Kitsune most often appears as a woman to human men or as a man to human women. Often wishes to merely play pranks, but sometimes desires to mate with humans. They are generally considered harmless. The Japanese people believed that the kitsune was the messenger of the Fox goddess Inari.
- Shikigami (式神, "Formula God"): Spirits that are summoned to serve or protect an Onmyoji, much like the European wizard's familiar. Shinto priests and some miko also are capable of summoning shikigami. Shikigami can appear as birds, small animals, or humans. The range of abilities possessed by a shikigami is dependent on the Onmyoji's capabilities.
- Tanuki (狸, "Raccoon-Dog"): the shapeshifting raccoon-dog apparition is fond of drink and food to excess: it often tricks humans by purchasing sake with leaves disguised as money. Tanuki are generally harmless tricksters - but their tricks seem to escalate if the one being tricked merits it. There is a legend about tanuki in Japan, concerning the origins of Inu-yama (Inu mountain) in Gifu prefecture, which was the home of Oda Nobunaga in 1575.
- Inugami (犬神, "Dog God"): A demonic race of creatures that resemble dogs with the ability to shapeshift from dog to human form.
- Yōkai fangs are shown to possess some supernatural powers on their own, if removed as long as the yōkai is still alive, such as the case with Goshinki, whose fangs were used to forge the Tōkijin.
- It is unknown whether there are demons exist outside Japan.
- As the Grim Comet and its fragments periodically bring yōkai to the Earth, it is heavily implied that at least some if not all demons are extraterritorial (or alien) in nature.
- Yōkai on Wikipedia