Eight Headed Orochi

The  is a legendary Yamata no Orochi which resides in the source of the Water of Life in the forests of Ryugenzawa. It was battled by Akane so she could get some moss to finally cure Shinnosuke from having to rely on using the Water of Life to survive.

History
The Orochi is a giant, eight-headed dragon serpent that sleeps at the bottom of the Spring of Life. The eighth and largest head actually forms its body, with the seven smaller heads positioned at the end of tail-like necks at the opposite end. This head and the seven smaller heads are shown to be capable of both independent thought and concerted effort. The magic moss that grows on the largest head when mixed with the lake water becomes the Water of Life, which makes the animals that drink it grow to monstrous proportions. This moss was also found to be capable of healing Shinnosuke's fatal wounds.

Ryugenzawa
Although it is normally dormant, a sign that the Orochi is awakened is when the numerous springs throughout Ryugenzawa begin to dry up, because the Orochi blocks the source of the water with its largest head. In the manga, the animals revert to their normal sizes because the Water of Life has been withdrawn from them, but this does not happen in the anime. The Orochi is a wine-lover and a pervert that loves to eat beautiful women, and will kill any male in its way. Despite its size, all the heads are very fast, including the largest head. The smaller heads can also breathe fire (portrayed more as energy blasts in the OVA) if sufficiently provoked. It is Ranma's most powerful non-human adversary, and was only defeated by Akane blowing the Horn Whistle, which made its heads drowsy and eliminated its will to fight. The Orochi then returned to the lake where it dived in and returned to sleep. The moss growing on it began to mix with the lake water again, restoring the Water of Life. The Orochi's voice effects were done by Toshihiko Nakajima.

Abilities
The Orochi is a true monster of an opponent, its huge size and thick scales making it extremely hard to hurt. It is impossible to say for sure if it could even be killed, though no serious effort to injure the creature was actually made in canon.

The Orochi has a single mountainous head that also forms it body, as well as seven smaller, but still huge, heads on the ends of long necks that sprout from the back of its primary skull. It is likely that all of its vital organs are actually contained in the primary head-body; as this is usually kept buried underground or submerged underwater, opponents are instead left only to attack its comparatively expendable tail-heads.

The Orochi has massive crushing force in its jaws and myriad sharp teeth. It can also exhale blasts of fiery energy as projectile attacks. Its tail-heads are able to strike with considerable speed and from multiple directions at once, making it extremely hard to fend off.

The Orochi's only weakness is that the whistling noise made by blowing into the horn taken from a horned mongoose, its "one true nemesis", sends the creature into a decades-long slumber.

A magical plant called the Moss of Life grows on the Orochi's body. This moss is full of powerful energies and can heal almost any wound. Even being immersed in water transmutes the water into "Water of Life", which also has substantial healing powers and can cause animals who drink it to grow into gigantic sizes.

Trivia
The Yamato no Orochi is a legendary dragon from Japanese mythology. Having eight heads and eight tails, it devoured maidens until the exiled storm god Susano-o used trickery to kill it. In one of its tails, he found a sword, which is now one of the Three Regalia of Japan.

In terms of appearance, this Orochi is vaguely similar to a Beholder, a monster from Dungeons and Dragons that has a crudely head-like body adorned with multiple eyes on stalks.