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.

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.