The Yamata-no-Orochi is one of the most famous mythical beings of Japanese mythology and one of the relatively few widely known examples of a truly evil dragon in Asiatic myth. Taking the form of a massive dragon or serpent with eight heads and eight tails, it actually acts a lot like a Western dragon, demanding sacrifices of virginal maidens for it to devour. Like a lot of famous named mythical beings, however, it actually only appears in a single story, which is the part of the myth of how the storm god Susano'o learned humility.
To cut a long story short, Susano'o was rampagining through the realms of his sister Amaterasu the sun goddess in Heaven, wreaking havoc as he went. The final straw came when he killed on of Amaterasu's horses, flayed it, then flung the skinless corpse into her palace's weaving room, where it either crushed one of her handmaidens to death or caused her to die of fright. This made Amaterasu run away and hide herself in a cave, plunging the Heavens and the Earth into darkness and forcing the other gods to have to coax her back out again - but that's it's own story. Once Amaterasu was back, the other gods kicked Susano'o out of Heaven and banished him to Earth.
Down on Earth, Susano'o came upon two Earth Kami weeping their own misfortune, who explained to him that for the past eight years, their holdings had been terrorized by the Yamata-no-Orochi, which had forced them to sacrifice one of their daughters every year in exchange for not destroying them - and now they were down to their last daughter; Kushi-inada-hime. In a bit of Classical Hero behavior, Susano'o offered to slay the beast if they would let him take their last daughter as his wife. They agreed, and he set out eight great barrels of strong liquor, waiting for Yamata-no-Orochi to drink them all and fall asleep before he summarily decapitated each of its eight heads. Then he chopped off its eight tails for good measure, finding the magical sword Kusanagi-no-Tsurugi in one of them; this sword became a sacred relic, one of the three Imperial Regalia of Japan that are traditionally born by the Emperor. In fact, the first Emperor of Japan was the firstborn son of Susano'o and Kushi-inada-hime.
Most modern interpretations of Orochi (and beings based off of it, there are a lot of both) mostly focus on the whole eight-heads element and tend to cast it as a great evil, if not greatest of them all.
Magic the Gathering
Orochi are a race of bipedal, four-armed serpentfolk native to the Magic: The Gathering plane of Kamigawa. Denizens of the Jukai Forest, they are a hunter-gather people segregated into three main tribes; the Kashi (warriors), the Matsu (scouts), and the Sakura (shamans). Despite their startling appearance, they are a friendly people who are on good terms with humanity, learning martial arts from the Budoka, the warrior-monks who cohabit their forest. Their shamanistic magic is based on their reverence of the land's kami (spirits) and is noticeably tied to the seasons. This gives the race a strong affinity for Green Mana.
Sadly, because of how badly the Kamigawa block was received, little else is known about the orochi. Which is a shame, because they would make a great race for an Oriental Adventures setting. Alas, their appearance in an article of Plane Shift is uncertain at best.