Back in September 2005, a long post was made to a Naruto fan-board, purporting to detail an ancient Japanese legend on which Naruto's author was supposedly basing his story. The author of the post refused to name any of her sources (although this apparently-fictional Chinese rambling from the month before is suspect), and wrote it up in a style much more reminiscent of a monster manual from a role-playing game than any ancient legend, but that didn't stop overzealous fans from promptly spreading this information to thousands of pages all over the internet, translating it into dozens of languages (including Japanese), incorporating it into their fan-fiction and writing up wikipedia articles on the creatures discussed therein based entirely on this guide. One poor misguided soul even declared himself a five-tailed "Houkou" otherkin, prompting a writeup of this noble beast on Youkaimura.
Unfortunately, it's all complete bollocks.
The author herself even admitted it, eventually, but not too many people seem to have noticed.
Here's some of the most egregious nonsense, masquerading as Real Folklore™:
二尾 | 貓又 – Two Tail (Nibi), Nekomata
Japanese Myth Appearance: A species of two tails black cat, with slightly more pointed ears and bigger canines.
All traditional images of Japanese monster cats I've seen have had the same white-with-yellow-and-black-patches coloration as your stereotypical maneki neko. But of course they can't be scary if they're not black!
三尾 | 磯憮 – Three Tails (Sanbi), Isonade
Japanese Myth Appearance: A big shark/fish(?) with 3 tails and 3 fins. Has a dark blue body color, is attracted by blood (like all sharks); has a personal servant, a small fish named Samehada, as shown above, that is responsible to help Isonade absorb and digest Chakra in 5 times his original capacity.
This is much worse. The Isonade does not have three tails, three fins, or three of anything. All it has is one huge tailfin covered in little hooked barbs which it uses to snare sailors so it can drag them into the sea and devour them. This "samehada" thing is completely made up. I personally rewrote the wikipedia article on this one, but the bogus version is still on every one of the hundreds of wikipedia clones. Wonderful!
四尾 | 鼠鮫 - Four Tails (Yonbi), Sokou
Japanese Myth Appearance: A four-tailed Rooster / Snake mixture. The lower part of the body is a Snake, while the upper part is a Rooster. Crawls instead of walking (Just like desert scorpions *COUGH*Hiruko!Sasori*COUGH*). Most of the body parts have an orange tone. Lives in the mount Fuji.
This one's even better. This one doesn't even exist! At least, not in Japanese. The kanji for its name seem to refer to a real life "rat shark", but they're read "nezumizame".
五尾 | 彭侯 – Five Tails (Gobi), Houkou
Japanese Myth Appearance: A species of dog, with 5 long tails. Has a white coloured body. Each of the tails possess one of the five elemental powers, that used simultaneously can cause a big earthquake.
Poor, poor Hōkō. You're not even a Japanese creature, you came out of a Chinese book called the Sou-shen-ji, and the Japanese only know about you because good old Toriyama Sekien drew a picture of you. You don't have five beautiful tails, in fact you have no tail at all, and you're black, not white, and your face looks like a man's. In fact your most notable feature is your delicious flavor when steamed, as you don't seem to put up much of a fight after your tree is cut down. At least I rewrote your wikipedia article for you.
六尾 | 雷獸 – 6 Tails (Rokubi), Raijuu
Japanese Myth Appearance: A species of weasel with golden yellow fur. The whole body’s fur grows upright. Its six tails take a lightning shape, (unlike the crappy image I have, but it’s the closest to the legend) just like the ancient God of Thunder.
This one I haven't done as much research on, but lightning-shaped tails? I think somebody's been watching too much pokemon. A quick look around the Japanese internet reveals that the raijū has got a whole lot of different descriptions, resembling various different small mammals, although one of them gives it six legs and three tails.
七尾 | 貉 – Shichibi (Seven Tails), Kaku
Japanese Myth Appearance: Kaku is the smallest Bijuu. It has a blue color, and is good at using clay camouflage. Kaku is very difficult to bump into.
The only reference I can find to a seven-tailed mujina is here: http://www.nichibun.ac.jp/YoukaiCard/65
It says it stole tea grounds that were stored under the floor of a soba shop in Edo.
八尾 | 八岐大蛇 - Eight Tails (Hachibi), Yamata no Orochi
(Also known as Hachimata)
Japanese Myth Appearance: A super Snake of Eight heads and Eight tails, attached to a single body. Has a deep brown color. It definitely is the biggest bijuu - the lenghth of it is over eight mountains. Originally, it was weaker than Houkou and Nekomata; however, that was 1000 years before a brave warrior tried defeating it with the Kusanagi no Tsurugi. In the process, the sword’s power is absorbed instead by Yamata no Orochi, thus greatly expanding his power. He continues though to be able to use freely the sword in his belly.
This is...pretty weird. The Yamata-no-Orochi indeed had eight heads and eight tails, and stretched over eight mountains and eight valleys, but it was defeated in the Kojiki by the god Susanoo, who got it drunk, hacked it into pieces, and found the Kusanagi-no-Tsurugi hidden in one of its tails.
What's even weirder is that this stuff evolved to the point that some genius on wikipedia decided that "Hachimata" was a distinct entity from the Orochi, which it is not - it just appears to be a bad reading of "Yamata". People even debated merging the two despite the fact that the description given in the Hachimata article was more-or-less exactly the same as the one given to the Orochi in the Kojiki. Do people even read things anymore before trying to write encyclopedia articles?
九尾 | 九尾の妖狐 – Nine Tails (Kyuubi), Kyuubi no Youko
Japanese Myth Appearance: Kyuubi no Youko is the strongest (S Level) Bijuu of the Japanese Myth. Its body has a red fur; Kyuubi represents the element of Fire. Its abilities are incredible. Because it has never been defeated by any youkai, its total strength is unable to estimate. His tails create cyclones by spinning quickly, and Kyuubi proceeds to rip its enemies with its huge claws. At the same time his fur may shoot fireballs similar to meteorites endlessly, instantly destroying villages. In the ancient Bijuu wars, a challenge results in Houkou being severely wounded, and Nekomata nearly dying, but ending up being rescued by his master, Shinigami (God of Death). Yamata no Orochi depended on the Kusanagi no Tsurugi to fight Kyuubi, but was defeated and the sword received a cracked hole, courtesy of our kitsune.
This has, as one might expect, the most fabricated crap in it. If you're familiar with the legend of Tamamo-no-Mae you probably know that all it takes to defeat a nine-tailed fox is a guy with an arrow - although you might need a priest later to clear its spirit out of a pile of poisonous rocks. How the Orochi could fight with a sword that was embedded in its tail after it was hacked to pieces is anybody's guess. It's also very interesting that the Nekomata's master is the Shinigami, which is really just the Japanese name for the Western Grim Reaper.