A beastman type monster with the characteristics of a fox. Beside living in forests and mountainous areas, many of them slip into human settlements with a man who strikes their fancy after charming and enthralling him.
Monsters are basically lustful, but the Youko stands out as being particularly lustful even among them. When they spot a man they like, they aggressively attack, having wild and obscene sex with him. They accumulate demonic energy on this occasion by obtaining spirit energy from the man. Their tails are a symbol of high demonic energy, the higher their demonic energy is and the more they have sex with men, the more they increase in number until they reach 9.
Also, the immensity of their lust is proportional to how many tails they have. With only a single tail, they're not much different from other monsters, but as their tails increase in number, their minds become occupied with thoughts of pleasure and sex with men. By the time they reach 9 tails, they end up thinking mostly about having sex with men, whether awake or sleeping. Their main concern is how to have sex with men and how to enjoy pleasure. They spend much of their time having sex with men except when sleeping.
Men whom they've taken a liking to that have started to live with them and periodically have sex with them will end up possessing high abilities and high lust due to the influence of their demonic energy. Due to these heightened abilities, it appears that those chosen to be their husbands are commonly found in high positions in human society, such as statesmen.
Individuals with 9 tails are called "Kyuubi-no-kitsune" (Nine tailed foxes) and it is said that their vast demonic energy is even comparable to that of a god. Their demonic energy can't be completely contained within their body and it's constantly being released around them. When exposed to this magical energy, human rationality diminishes and lust increases. Monsters become ferocious and along with that, monster such as succubi gather. Ultimately a nation of monsters will form centered around the Kyuubi-no-kitsune, where they'll have an orgy of pleasure day and night.
They're also called "keikou-no-bijo" (women so beautiful they can steal a ruler's heart and put the nation in danger) and it's certainly true that they will cause human order to crumble and a country to transform, with the order of the monsters, consumed by pleasure focused around them and their husband, replacing it.
In the uncivilized "Mist continent," it is said that kyuubi-no-kitsune rule and a devilish paradise that we can say is truly debaucherous is spreading, where humans and monster are consumed with thoughts of having sex according to their desires, they can enjoy pleasure and copulation all day every day whether awake or sleeping.
- As opposed to most Beastmen mamono, a Youko's Demonic Energy generally creates a Dark Demon Realm as opposed to a Bright Green one.