Oni
Oni are large, often thuggish, immensely powerful spirits that typically dwell in the mountains and/or hells of Japanese and Chinese mythology. Although vaguely similar to the Ogres of the West, Oni are more morally neutral as a whole; dangerous and unpredictable, but not usually evil, and some are even good. Japan is famous for having many monstergirls in its mythology, and oni are no exception; female oni are said to be very beautiful, and even happily marry mortals, but they retain the immense strength and magical powers of their male counterparts, and are said to be capable of making themselves hideously ugly - as in, kill you with fear at the sight of them ugly - when angry.
Oni generally look like giant humans, often with odd skin colors, who may have odd numbers of eyes (1 or 3), 0-3 horns on their heads, fangs and/or tusks. In Japanese mythology, red-skinned oni are commonly said to be impulsive, extroverted, wild and emotionally driven, whilst blue-skinned oni are commonly said to be patient, introverted, calm and logical. Hence the trope for contrasting an emotional character vs a logical one commonly being "Red Oni, Blue Oni".
Oni have a long tradition in Dungeons & Dragons, where they were initially ported over as the "Japanese Ogre", which eventually became the Ogre Mage in AD&D. This was described as an ogre, but much smarter, and with both a variety of innate magical abilities and an affinity for spell-casting classes. The Dragon Magazine article "Ecology of the Ogre Mage" in issue #349 gave them a pair of sub-species; the psionic Cereborgs, and the Ogre Umbramages, who replaced the usual spell-like abilities with darkness and invisibility at will, and detect thoughts, enervation, shadow walk and waves of fatigue once per day, and were described as favoring classes like the ninja and shadowcaster.
An AD&D Oni did appear in the Kara-tur setting, as can be seen here: http://www.lomion.de/cmm/oni.php
In 4th edition, WoTC decided it was time to stop denying that the Ogre Mage was anything other than an Oni with the name changed. They created an entire class of oni creatures, with the Ogre Mage now the... Oni Mage... and added the Night Haunter in their first MM. This creepy sumbitch had a breath weapon that could put victims to sleep, heal itself by draining life from sleeping victims, disguise itself as any elderly humanoid of Medium or Large size, and assume a gaseous form. The second 4e MM added the Devourer, Overlord and Thunderer oni.
Pathfinder, meanwhile, took a leaf out of WoTC's book and changed the Ogre Mage into just one breed of an Oni category. Their Oni became a species of evil outsiders, envious spirits whose sheer hedonistic desire caused them to take on the form of humanoids so that they could experience physical sensations again. The Ogre Mage Oni was still pretty vanilla, but the 3rd Pathfinder Bestiary, and the Jade Regent adventure path, added a bunch more oni.
The Fire Yai imitates a Fire Giant, can launch a fiery beam from its third eye, and turn itself into smoke. Craving luxuriant dwelling places, they are the most impulsive of their kind, a reference to the "red oni" mythology mentioned above.
The Ice Yai imitates a Frost Giant, can launch a freezing beam from its third eye, and has a monk-ish Staggering Strikes ability. Egotistic and domineering, they are nonetheless a very disciplined, dedicated, and even-tempered breed, preaching that the threat of violence can be more effective than actual violence. Like the Fire Yai, this taps into the "blue oni" mythology.
The Kuwa is the weakest and most common kind of oni, being based on the human form. The result is a claw-fingered, golden-skinned humanoid with fairly meager magical powers, as oni go. They crave power, but are more cautious than most other oni; they tend to avoid positions of true authority like king or mayor in favor of hiding in the upper echelon, where they can more freely exploit and abuse their powers to live in wealth and luxury.
The Spirit Oni are the weakest of all oni, an "unborn" oni that has been magically coaxed, tricked or otherwise sealed into a mask, which can then be used as a symbiote in order to call upon the oni's magical power. They are pitiful, hateful creatures who loathe being so entrapped, but which are too cowardly to take the risk of destroying themselves so that they can try to reincarnate as a more powerful oni.
The Void Yai is the most powerful form of oni, a towering dusky-skinned giant-form with, among other things, the ability to launch a negative energy beam from its third eye, assume an ethereal "void form", and create a lessened Sphere of Annihilation by use of its teleport powers. The most ambitious of all oni, Void Yai seek to conquer kingdoms to serve them. They hate their true forms as ugly, preferring to disguise themselves as incredibly attractive giants instead.
The Water Yai imitates a Storm Giant, wears unique magical robes, can shoot acid from its third eye, and turn into water. They love beauty, especially beautiful treasure, and seek to surround themselves with it. Noted for being surprisingly frivilous, especially for an oni.
The Yamabushi imitates a tengu, appearing as a crimson-faced humanoid with a big nose and raven-like wings. Proficient with monkly weapons, able to steal voices, but vulnerable to Pattern sub-school Illusion spells, they are noted for their love of trickery and thieving, driven by an urge to collect all the shiny treasure they can. Also, they hate ducks. Seriously, they find ducks so ugly and disgusting that they will actually endanger themselves in combat for a chance to kill a duck. Detailed in Jade Regent #1.
The Nogitsune imitates a kitsune, and possesses a unique form of the Suggestion spell-like ability that can actually be spread to new victims through the speech of those under its command. Hedonists who love killing and fleshy pleasures, they tend to become assassins and spies for both the love of murder & mischief and to get the money to spend on whores, booze and drugs. Detailed in Jade Regent #2.
The Atamahuta imitates an ettin, the notorious two-headed ogre. Distinguished by the way its left head has three eyes and constantly gibbers to itself, the Atamahuta divides its anger & savagery into the right head and its ingenuity and magical prowess into the left. Consequently, it talks through the right head, but casts spells with the left. As a result, it can easily enchant its weapons and is hard to disrupt its casting, as the body can keep protecting itself even as one head casts spells. They crave power and fearful respect above all else, so it incenses them that they often have to find a humanoid to serve as their "face", since their ettin and oni forms alike earn the repulsed disdain of the mortals they want to dominate. Detailed in Jade Regent #3.
The Ja Noi imitates a hobgoblin, but has a hunched back, fiery red skin, and over-developed muscles. It craves the carnal thrill of slaughter, finding the true pleasures of life to be found in the spilling of blood, the shattering of bone, and the thrill of life-risking combat. They yearn for battle and constantly seek out worthy opponents to test their might on; they rarely live long, especially because the followers they amass eventually realise they are expendable to a Ja Noi in its desire for bloodshed, but they see that as just the more reason to live to the utmost whilst they can. Detailed in Jade Regent #4.
The Earth Yai imitates a Stone Giant, can launch boulders from its third eye and grow protective stone spikes from its skin. It craves power above all else, yearning to hold the ultimate ability to inflict ruin. They are smart enough to realise that this doesn't necessarily entail physical strength; a soldier is less destructive than the catapult shot that brings down a castle wall, which is less powerful than the king whose decree ignited this bloody war in the first place. Detailed in Jade Regent #5.
The Wind Yai imitates a Cloud Giant, can fire lightning bolts from its third eye, and has powers relating to control over the wind and weather. Brooding, blustering fiends who desperately want to control the weather with the grace of the Cloud Giants they try to imitate, but who fail due to their lack of delicacy, which incenses them. Detailed in Jade Regent #6.