Every one in a while, one will come across an anime with strange creatures that have a spiritual nature to them. These Yōkai (妖怪, "strange apparitions") appear all throughout Japanese folklore. Many anime fans are familiar with terms like Kyūbi, Kitsune, and Tengu; these creatures all qualify as yōkai. There are plenty of other instances of yōkai throughout anime.
It's important to remember that yōkai come in all shapes and sizes. While many of them are ghosts or at least spiritual in nature, they represent myths and rumors in general for the Japanese. As such, they can be anything from a woman with her mouth cut open (Kuchisake-onna) to giant sea monsters (Umibōzu). In this sense, even onis, despite being called demons or ogres in English, qualify as yōkai. It's better to think of them as fairy tale characters if anything. With that in mind, here's a look at some of the biggest anime that feature them.
One of the most famous yōkai anime out there is GeGeGe no Kitarō. It follows the eponymous Kitarō as he befriends and battles new yōkai on a regular basis. Dating back to the 1960's, Kitarō could be called the progenitor of modern supernatural anime. It's definitely responsible for popularizing yōkai for modern anime audiences. However, due to its long history, it might not be the best place to start for those with only a passing interest in yōkai.
A better place to begin would be Yōkai Watch. The anime has its main characters interact with, fight and befriend various yōkai. This series plays around a lot with the idea that yōkai are responsible for unexplained phenomena. It's like Pokémon, but with yōkai.
Speaking of Pokémon, that franchise also features a few yōkai-like monsters, and it's not just the Ghost-types, either. Naturally, Ninetails is based on the popular Kyūbi. Other fox-like Pokémon like the Zoroark line, the Fennekin line, and Vulpix may be based on the Kitsune. Mawile is based on the futakuchi-onna. Golduck and Lombre are based on Kappa. Froslass is based on the Yuki-onna. The list goes on and on. For series like these where yōkai wouldn't logically exist but are still a desirable plot device, such substitutions become necessary.
As it turns out, the recent Wano Arc of One Piece features a number of yōkai-like entities, especially during the Raid on Onigashima. Several of the Homies Big Mom made on the island are based on tsukumogami, spirit-possessed objects which are often classified as yōkai; these include Bakezōri (animated zōri sandals), Chōchinobake (animated lanterns), and Kasa-obake (animated umbrellas). Black Maria of the Tobi Roppo is based on a mix between a Jorōgumo and a tsuchigumo, two types of spider yōkai; her underlings in the Pleasure Hall are also based on various yōkai, including a Rokurokubi, an Akaname, and a Nure-Onna. It's also possible that Kaidō and Yamato are onis, though the details on that front are still foggy; at the very least, they're obviously based on onis. This dense collection of yōkai references is likely the result of Wano being inspired by Japan.
A common trend in anime featuring yōkai is to exorcise them, which makes them perfect for the battle shōnen genre. As such, many series from Shōnen Jump prominently feature characters fighting yōkai, evil spirits, Mononoke, demons, or whatever they want to call them. Popular supernatural anime under the Shōnen Jump banner include Jujutsu Kaisen, Yū Yū Hakusho, Hell Teacher Nūbē, and Nura: Rise of the Yōkai Clan. Some lesser-known ongoing series include Ayakashi Triangle, Ayashimon, and Doron Dororon. These kinds of series will often depict yōkai as malevolent beings who terrorize ordinary people; it's the main characters' duty to stop such evil spirits and maintain peace.
Like yōkai, anime featuring them can come in all shapes and sizes. Other popular series that prominently feature these strange creatures include Inuyasha, xxxHolic, Natsume's Book of Friends, Ghost Stories, Noragami, and Sewayaki Kitsune no Senko-san. One could watch some or all of these series and make a real Hyakki Yagyō (百鬼夜行, "Night Parade of One Hundred Demons") out of their evening.
The folkloric nature of yōkai is what makes them so appealing. They can be anything from horrifying monsters to playful sprites. They can be humanoid or they can be bestial. Some of them can even be cute and lovable. It's all about who's telling the story, which is what makes yōkai perfect for anime.
