Only technically "beasts", Hanjuu are half-beasts with the intellects and sentience of humans. They can change between their human and beast forms at will.
Anime-only character. A nameless cat hanjuu on a ship Asano rides on.
Kantai is half bear, half human. See Kantai.
Rakushun is half rat, half man. See Rakushun.
Kijuu are tamed youjuu.
Nyokai are creatures that are between humans and youjuu, and are therefore called nin'you (人妖) or youjin (妖人). Only the special ones born on Mt. Hou are called nyokai. For every kirin there is a nyokai. The nyokai are the guardians of their respective kirin from even before their birth, while they are still in the ranka. Their ranka grow on the roots of the Shashinboku (the riboku tree on Mt. Hou) while the ranka of their kirin grow on the branches. Both the nyokai and kirin ranka are golden in color, a stark contrast to the typical human red. The Nyokai live to serve the kirin.
The unnamed nyokai of the yet-to-be-born new kirin of Kou.
The sacred beast that guards Reiken Gate. It has a strange appearance that can be termed neither youma nor youjuu. It has the body of a dragon, with large wings. Its voice come out sounding like it's growling, and it can take on human shape. Its human form looks like an old man with his limbs bound by black steel. He opens the Reiken Gate. It is rumored that it likes in the rousuu.
Shinjuu are divine beasts.
A divine beast which resembles a turtle and is about the size of a small island. After the coronation of a ruler, the ruler and kirin ride Genbu to their kingdom's palace.
Kirin are similar to hanjuu in that they have both human and beast forms. However, their beast form is invariably that of the mythical kirin (which greatly resembles a shaggy unicorn). They are holy creatures, and there can be no more than twelve of them at a time—one to serve each of the twelve kingdoms. They, and they alone can choose the rulers of the twelve kingdoms.
Kirin are herbivores by nature. They dislike conflict, and exposure to blood—even their own—will make them ill. Prolonged exposure, such as that suffered by Taiki in Tasogare no Kishi, Akatsuki no Sora, can bring them close to death.
Though some of them are given nicknames by their rulers, and they also hold the titles of Taiho and Saiho, the most commonly used name for kirin (interkindom) is developed through combining their country name (i.e. "Kou" or "Kei") with the suffix of "rin" (from kirin) for females, or "ki" (also from kirin) for males. (e.g. Kourin or Keiki).
Shirei are youma that have contracted themselves to protect and serve a kirin on the condition that when the kirin dies, the shirei get to eat their remains. (Apparently, kirin are very tasty. No, actually, the remains of the kirin still retain of the kirin's powers. The shirei then eat the remains, and because of the contract they had with the kirin, they are able to absorb that power.) Shirei have the ability to command others of their kind should a kirin command it of them.
An isoku youma
A kogou youma.
A hiso youma. Jakko is only technically a shirei, as it does not really have the power to be a shirei. Keiki only tamed Jakko as an example when he was teaching Taiki about getting shirei.
A hinman youma.
A youwa youma.
A kiki youma.
The toutetsu Taiki binds, which after its binding takes the form of a small white dog.
While youma are fantastical demons, youjuu are fantastical beasts. In theory, anyone is capable of taming a youjuu, while only the holy kirin are capable of taming youma, as youma naturally have a far more vicious disposition. Tamed youjuu are called kijuu, while tamed/contracted youma are shirei. Youjuu cannot be made into shirei. Youjuu have no riboku tree from whence they are born and it is said that there are no infant youjuu. Although there are both male and female youjuu, females are extremely rare. The youjuu feed off of youma meat, though because they are often hunted (probably to be tamed into kijuu) many of them attack humans. Ayakashi (妖) is a general term used to refer to both youma and youjuu.
Horse-like youjuu that have a single, sharp horn, and thick claws instead of hooves. Their skin shines silver when hit by light. They can fly long distances faster than boats can travel them. They are omnivorous, and their voices are several pitches higher than that of horses.
Youjuu that resemble horses. They have red manes and their bodies have white stripes. They can be tamed to fly as kijuu.
Youjuu that resemble white leopards, with golden eyes and a white tail. They are intelligent and mild-manned. They often try to understand people, and become emotionally attached to their masters. When someone calls to them they will go to them without thinking too deeply about it. They are not good fliers, but whether by land or in the air, they are three times faster than horses.
Blue, horse-like youjuu that cannot fly. They run out of breath quickly and though not very fast, are three times faster than horses. Suzu buys a sansui in her desire for revenge against Shoukou.
Tiger-like youjuu suited for combat. They give off the impression of being very fierce and strong. They have complex, pearl-esque black eyes and black and white stripes. Their tails are as long as their bodies, and their fur is harder than it looks. They are the fastest of all kijuu and are very brave, determined, and intelligent. They are hard to get a hold of. Fast, bold, and sly, they like agate. They can cross a entire kingdom in single day. If you captured and tamed one, you could live the rest of your life off what you sold it for.
Youma are basically demons. Many of them eat humans and they are greatly feared by the citizens of the Twelve Kingdoms. If youma are prowling around, that is a sign that that kingdom is becoming unstable due to either the abscence or instability of the monarch. The only beings capable of taming youma are the kirin. The kirin contract with the youma so that the youma protect and serve the kirin, and in return are allowed to devour the power-ridden remains of the kirin when he/she dies. Ayakashi (妖) is a general term used to refer to both youma and youjuu.
A rabbit-like youma with short ears, or it could be a big mouse-like youma with a long, thin body. "Hiso" literally means "flying rat."
A youma that resembles a giant chicken, but with a poisonous tail shaped like a small knife.
A type of flying tiger, Kyuuki are fierce, man-eating youma. In the series, a Kyuuki attacks the rike Youko stays at.
A snake with the stature of a man. It has rainbow-colored scales covering its stomach, three legs and four wings.
An enourmous red-haired ape with white fur around its neck. It has pointed teeth, claws like those of raptors and is capable of smiling.
An enourmous, red, wolf-like youma. Its name literally means "heavenly dog." Rokuta is a tenken.
Those that don't fit in any other category.
During its lifetime, a hakuchi cries out with a human voice only twice: once during the coronation of an emperor, and once when the emperor dies. Consequently, it also sometimes is called "nisei" (二声、"two voices"). When it finishes its second call it dies on the spot. In the abscence of an emperor, it is customary to use the leg and foot of the dead hakuchi in place of the Imperial Seal.
A pair of birds in each kingdom. The hou is male and the ou is female. The ou of one country can communicate with the ou of another. The hou cries when the ou receives important news.
A blue bird that government officials (and anyone else who can afford them) use to communicate with officials in other kingdoms.
References: Twelve Kingdoms/Juuni Kokki page youma list and encyclopedia, as well as Eugene Woodbury's blog. 12k-TP's The Twelve Kingdoms Fansite anime notes also helped fill in a few gaps, and I'm indebted to 十二麒麟・juunikirin and 十二国記の部屋 for a lot of the kanji, as well as a great deal of info.