OniProbably the most fearsome Youkai in all of Gensokyo. Known mainly for their overwhelming physical strength, it takes Oni extermination specialists to deal with these fearsome ogres, and those are increasingly less common in Gensokyo. They're easily identifiable by the horns on their heads and their huge muscular stature. Oni love drinking and partying, but they also like to challenge humans to competitions. If you lose, you're abducted, and the Oni cannot be beat in anything. That said, Oni are known to be very honorable and hate cowards and liars.
Do not face an Oni without a specialist with you. Do not accept an Oni to his challenge. Do not look for an Oni. Do not insult an Oni. Do not even get near an Oni, and for the love of all that is holy, do not drink with an Oni. Just remember that they are weak to fried beans.
The Oni have lived in Gensokyo for a long time, having established their own society upon Mt. Yatsugatake (otherwise known as Youkai Mountain). Every now and then, they'll come down and kidnap a human, and it's up to a specialist to deal with them. The leaders of said society are known as the Four Devas of the Mountain, and they are the toughest among all the Oni (two of them are pictured above.) They rule Oni society with an iron fist. It appears that the Tengu and the Kappa are subordinates to the Oni, although it appears that not all of them seem all that happy about it. Indeed, there is unrest between the three societies on the mountain.
TenguSome know them as the fastest Youkai, the Tengu are spirits of the wind. Their forms vary between crow-like forms to monks with long noses and red faces, but they all share very similar traits. They have the natural ability to fly and the ability to control the wind using their hauchiwa fans.
Tengu are known to be ancient and have developed a very strict hierarchical society. Perhaps it is because they can transfer information between each other faster than humans can (note: although now, there exists something called a telegram) that they have the most well-organized society of all. In Gensokyo, however, Tengu live mainly around the peak of Mt. Yatsugatake and are merely subordinates to Oni and serve as their messengers, guards and scouts. It seems, however, that they might be getting fed up with it.
KappaAlthough they aren't as ugly as most old stories depict them as, Kappa aren't any less unusual. Kappa are known to be water spirits and most commonly inhabit lakes. Do take note that they don't actually live in the water, but spend a lot of time near it. They are extremely strong swimmers, stronger than any human, and are known in stories to be the cause of many drowning incidents. Many stories tell of them abducting children or farm animals. However, they are known to be both very respectful and very gullible. They also love cucumbers. If you want to make peace with a Kappa, be sure to offer one of these and they won't abduct your child.
Kappa supposedly have their own hidden society somewhere at the base of Mt. Yatsugatake, but currently, they are subordinates to the Oni. They appear to have no real ill-will toward humans, however, and are quite curios about human culture. Western technology definitely seems to interest them. One shouldn't trust them, however, as they are both very introverted and quite mischievous at times. Among Youkai folktales, the Kappa may be known to be the easiest to deal with, but that doesn't mean they're any less potentially dangerous than any other Youkai.