The Yamata no Orochi, sometimes referred to simply as the Orochi, was a giant serpent that had eight heads and eight tales. Its body was so large that it could cover eight valleys and eight hills. Moss grew all over the creature's body. It was similar in appearance to the Greek hydra, although the Orochi is depicted as more intelligent than the hydra. Whereas the hydra is commonly depicted as completely beastlike, the Orochi is portrayed as being as intelligent as a person. There was only one Orochi in Japanese mythology. Here is the story of how the Orochi was slain:

Every seven years, the Orochi forced two elderly gods to sacrifice one of their eight daughters to it. By the time Susanoo, the storm god, met the two elderly gods, they have already been forced to sacrifice seven of their eight daughters and are about to be forced to sacrifice their last one. Susanoo agreed to slay the Orochi in return for the daughter's hand in marriage. The elderly gods agreed to his terms, so Susanoo turned their daughter into a comb that he put in his hair for safekeeping. 

Susanoo had the elderly gods build a large sense with eight gates. They placed a vat full of strong sake on a platform on top of each gate. Then, they waited for the serpent to arrive.

When the Orochi arrived, it drank all the sake. Shortly afterward, it fell into a deep, drunken sleep. Susanoo used this opportunity to slice the massive serpent into tiny pieces with his sword. The resulting carnage was so great that the Hi River ran red with the Orochi's blood.

As he was cutting the Orochi's fourth tail, the edge of Susanoo's sword broke. Upon closely examining the tail, Susanoo found another sword within it: the legendary katana known as Kusanagi-no-Tsurugi (meaning grass-cutting sword).

FUN FACT: Along with the Yata no Kagami mirror and Yasakani no Magatama jewel, the Kusanagi-no-Tsurugi is one of the three sacred Imperial Regalia of Japan. The exact locations of these three legendary items are unconfirmed, but the Kusanagi-no-Tsyrugi is thought to be at the Atsuta Shrine in Nagoya, the Yata no Kagami is thought to be at the Ise Grand Shrine in Mie Prefecture, and the Yasakani no Magatama is thought to be Imperial Palace in Tokyo. These three items represent the three primary virtues: valor, wisdom, and benevolence respectively. Here is what artists believe these three items look like:

Note that the following examples will depict both the Orochi itself and characters that are at least partially inspired by the creature.

Examples in various media:

Orochi (Blue Seed)

Orochimaru (Naruto)- Orochimaru can transform into a form resembling the Orochi by using the Eight Branches Technique.

Yamata Dragon (Yu-Gi-Oh!)

Orochimon (Digimon Tamers)