Yamata no Orochi

, or simply , is a legendary eight-headed and eight-tailed Japanese dragon/serpent.
Mythology.
Yamata no Orochi legends are originally recorded in two ancient texts about Japanese mythology and history. The 680 AD transcribes this dragon name as and the 720 AD writes it as . In both versions of the Orochi myth, the Shinto storm god Susanoo (or "Susa-no-O") is expelled from Heaven for tricking his sister Amaterasu, the sun goddess.
After expulsion from Heaven, Susanoo encounters two near the head of the , now called the , in Izumo Province. They are weeping because they were forced to give the Orochi one of their daughters every year for seven years, and now they must sacrifice their eighth, , who Susanoo transforms into a for safekeeping. The tells the following version:
So, having been expelled, [His-Swift-impetuous-Male-Augustness] descended to a place [called] Tori-kami (, now ) at the head-waters of the River Hi in the Land of Idzumo. At this time some chopsticks came floating down the stream. So His-Swift-Impetuous-Male-Augustness, thinking that there must be people at the head-waters of the river, went up it in quest of them, when he came upon an old man and an old woman, – two of them, – who had a young girl between them, and were weeping. Then he deigned to ask: "Who are ye?" So the old man replied, saying: "I am an Earthly Deity, child of the Deity Great-Mountain-Possessor. I am called by the name of Foot-Stroking-Elder, my wife is called by the name of Hand-Stroking Elder, and my daughter is called by the name of Wondrous-Inada-Princess." Again he asked: What is the cause of your crying?" [The old man answered] saying: "I originally had eight young girls as daughters. But the eight-forked serpent of Koshi has come every year and devoured [one], and it is now its time to come, wherefore we weep." Then he asked him: "What is its form like?" [The old man] answered, saying: "Its eyes are like , it has one body with eight heads and eight tails. Moreover on its body grows moss, and also chamaecyparis and cryptomerias. Its length extends over eight valleys and eight hills, and if one look at its belly, it is all constantly bloody and inflamed." (What is called here is the modern [winter-cherry]) Then His-Swift-Impetuous-Male-Augustness said to the old man: "If this be thy daughter, wilt thou offer her to me?" He replied, saying: "With reverence, but I know not thine august name." Then he replied, saying: "I am elder brother to the Heaven-Shining-Great-August-Deity. So I have now descended from Heaven." Then the Deities Foot-Stroking-Elder and Hand-Stroking-Elder said: "If that be so, with reverence will we offer [her to thee]." So His-Swift-Impetuous-Male-Augustness, at once taking and changing the young girl into a multitudinous and close-toothed comb which he stuck into his august hair-bunch, said to the Deities Foot-Stroking-Elder and Hand-Stroking-Elder: "Do you distill some eight-fold refined liquor. Also make a fence round about, in that fence make eight gates, at each gate tie [together] eight platforms, on each platform put a liquor-vat, and into each vat pour the eight-fold refined liquor, and wait." So as they waited after having thus prepared everything in accordance with his bidding, the eight-forked serpent came truly as [the old man] had said, and immediately dipped a head into each vat, and drank the liquor. Thereupon it was intoxicated with drinking, and all [the heads] lay down and slept. Then His-Swift-Impetuous-Male-Augustness drew the ten-grasp saber, that was augustly girded on him, and cut the serpent in pieces, so that the River Hi flowed on changed into a river of blood. So when he cut the middle tail, the edge of his august sword broke. Then, thinking it strange, he thrust into and split [the flesh] with the point of his august sword and looked, and there was a great sword [within]. So he took this great sword, and, thinking it a strange thing, he respectfully informed the Heaven-Shining-Great-August-Deity. This is the Herb-Quelling Great Sword. (tr. Chamberlain 1919:71–3) 
The also describes Yamata no Orochi: "It had an eight-forked head and an eight-forked tail; its eyes were red, like the winter-cherry; and on its back firs and cypresses were growing. As it crawled it extended over a space of eight hills and eight valleys." The botanical names used to describe this Orochi are or (winter cherry or Japanese lantern, "Physalis alkekengi"), (club moss, "Lycopodiopsida"), (Japanese cypress, "Chamaecyparis obtusa"), and (Japanese cedar, "Cryptomeria").
The legendary sword , which came from the tail of Yamata no Orochi, along with the mirror and jewel, became the three sacred Imperial Regalia of Japan.
Etymology.
The Japanese name derives from Old Japanese (with a regular from shift, but its etymology is enigmatic. Besides this ancient reading, the kanji, , are commonly pronounced , "big snake; large serpent".
Carr (1990:169) notes that Japanese scholars have proposed "more than a dozen" < etymologies, while Western linguists have suggested loanwords from Austronesian, Tungusic, and Indo-European languages. The most feasible native etymological proposals are Japanese from , (which is where Susanoo discovered the sacred sword), , or ; and , meaning "god; spirit", cognate with the river-dragon. Benedict (1985:167) originally proposed "large snake" was suffixed from Proto-Austro-Japanese *"(w)oröt-i" acquired from Austronesian *"[q]uḷəj" "snake; worm"; which he later (1990:243) modified to *"(u-)orot-i" from *"[q,ʔ]oḷəj". Miller (1987:647) criticized Benedict for overlooking Old Japanese " 'tail' + suffix "-ti" – as well as an obvious Tungus etymology, [Proto-Tungus] *xürgü-či 'the tailed one'", and notes "this apparently well-traveled has now turned up in the speculation of the [Indo-European] folklorists (Littleton 1981)." Littleton's hypothesis involves the 3-headed monster Trisiras or Viśvarūpa, which has a mythological parallel because Indra killed it after giving it soma, wine, and food, but lacks a phonological connection.
Mythological parallels.
Polycephalic or multi-headed animals are rare in biology, but commonly feature in mythology and heraldry. Multi-headed dragons, like the 8-headed Orochi and 3-headed Trisiras above, are a common motif in comparative mythology. For instance, multi-headed dragons in Greek mythology include the 9-headed Lernaean Hydra and the 100-headed Ladon, both slain by Heracles.
Two other Japanese examples derive from Buddhist importations of Indian dragon myths. Benzaiten, the Japanese name of Saraswati, supposedly killed a 5-headed dragon at Enoshima in 552 AD. Kuzuryū (, "9-headed dragon"), deriving from the Nagaraja snake-kings Vasuki and Shesha, is worshipped at Togakushi Shrine in Nagano Prefecture. (Compare the , "nine-headed bird" in Chinese mythology.)
Comparing folklore about polycephalic dragons and serpents, 8-headed creatures are less common than 7- or 9-headed ones. Among Japanese numerals, or () can mean "many; varied" (e.g., , "greengrocer; jack-of-all-trades"). De Visser (1913:150) says the number 8 is "stereotypical" in legends about kings or gods riding dragons or having their carriages drawn by them. The slaying of the dragon is said to be similar to the legends of Cambodia, India, Persia, Western Asia, East Africa, and the Mediterranean area." Smith (1919:215) identifies the mythic 7- or 8-headed dragons with the 7-spiked Pteria spider shell or 8-tentacled octopus.
The myth of a storm god fighting a sea serpent is itself a popular mythic trope potentially originating with the Proto-Indo-European religion and later transmitted into the religions of the ancient Near East most likely initially through interaction with Hittite speaking peoples into Syria and the Fertile Crescent. This motif, known as (German for "struggle against chaos") represents the clash between order and chaos. Often as these myths evolve from their original source, the role of the storm god (himself often the head of a pantheon) is adopted by culture heroes or a personage symbolizing royalty. In many examples, the serpent god is often seen as multi-headed or multi-tailed.
The fight of a hero (sometimes of extraordinary birth) against a dragon who demands the sacrifice of maidens or princesses is a widespread tale. In folkloristics it falls under the Aarne–Thompson–Uther index type ATU 300, "The Dragonslayer".