The One-Tailed Beast is one of the Land of Mountains Biju. It is a giant long-necked beast that dwells in a deep lake in the northern parts of the land. It is a creature believed to inhabit, specifically, Lake Yama-Tora in the Land of Mountains' Highlands. It is similar to other supposed lake monsters in the area and elsewhere in the land, though its description varies from one account to the next. Popular interest and belief in the animal has fluctuated since it was brought to the world's attention in early days of Yamagakure 2.
The earliest report of a Tailed beast associated with the vicinity of the lake appears in an ancient shinobi scroll recorded by the Tsurugi Clan, written sometime during the First Great Shinobi War. According to the writer, writing about a century after the events he described, the shinobi monks of Land of the Sun was staying in the land of the Mountains when they came across the locals burying a man by the river. They explained that the man had been swimming the river when he was attacked by a "water beast" that had mauled him and dragged him under. They tried to rescue him in a boat, but were able only to drag up his corpse. Hearing this, the Head Monk stunned the villagers by sending one of his followers to swim across the river. The beast came after him, but the Head monk made the hand sign of the cross and commanded: "Go no further. Do not touch the man. Go back at once." The beast immediately halted as if it had been "pulled back with ropes" and fled in terror, and both the monk's men and the pagan villagers praised God for the miracle.
Believers in the One-Tails often point to this story, which notably takes place on the river rather than the lake itself, as evidence for the creature's existence as early as the the First Great War.
Unlike its giant-turtle cousin, One-Tail is a slow swimmer. It usually cruises slowly below the surface of the water, using its long flexible neck to move its head into position to snap up unwary fish. Their four-flippered swimming adaptation may have given it exceptional maneuverability, so that they could swiftly rotate its bodies as an aid to catching prey.
What abilities this creature possesses remains unclear. However, it has been shown that its skin is very hard, able to shatter crystal and Diamond Release techniques with ease, it could also unleash a powerful roar attack that can repel matter, the roar also appears to have some relation to sonics as it could shatter crystals due to the high frequency, finally it has a 360° tidal wave technique that can cause large scale damage. One other ability that One-Tail has been shown to use was the opening of a portal to a separate dimension within the lake that it had appeared from. Whether or not this ability is exclusive to the One-Tailed beast or if the other Tailed creatures of the Land can use it as well is unknown. It has also not been revealed if the Tailed beasts reside within their own dimension together or separated.
Lake monster or loch monster is a term referring to purported fresh-water dwelling megafauna appearing in mythology, rumor, or local folklore, but whose existence lacks mainstream scientific support. A well known example is the Loch Ness monster. Lake monsters' depictions are often similar to some sea monsters. They are principally the subject of investigations by followers of the pseudoscience of cryptozoology.