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It must be confusing; if you try to translate them into English,they all translate as "demon/devil." But they are all different things. Yasha originally means a bad god in Buddhism, and we nowadays use this term only in combination with other words, such as Konjiki-Yasha (the title of a famous story) or Inu-Yasha (the title of a manga). It is rather old-fashoned term and used in literature. Oni is the most familiar imaginary creature in Japanese old tales. You can see the images of Oni when you Image-google the word 鬼. Oni look like humans, but have one or two horns on their head, wear fur pants, and usually they are red, blue, or some other color. Oni are tought to live in a remote mountain or on an island, and sometimes come to the village to do bad things like robbery or eating young women. Some historians think that Oni were the Caucasian people with blue eyes or red hairs who happened to come to ancient Japan, and Japanese people at that time were so astonished to see blue eyes that they create the image of Oni with blue or red colors. Youkai are verious types of imaginary creatures. There are more than 100 types in total. They are said to live near to us, and sometimes do bad things or tricks to human. For example, Karakasa-obake is an umbrella with an eye and a big tongue and a leg. It jumps around our house, and when he encounters a human, he shows his big tongue and surprises the man. Another youkai is Ittan-momen, which is like a long piece of cotton cloth flying and sometimes squeeze human neck. Or still another is Umi-bouzu, which is a huge black head appears in the sea and attacks a boat, etc., etc. Youkai is familiar to us by a famous manga called "Gegege-no-Kitarou." Akuma, on the contrary, does not stem from the Japanese tradition. It is the translation of Demon or Devil in the western culture, so the image of Akuma is almost the same as the demon/devil. Hope it helps.