In the Land of Mountains, there are ten giant demons called the Land of Mountains Tailed Beasts (尾獣, Yama no Kuni Bijū) that inhabit the land (these are separate beasts from the original Bijuu). They, like their counterparts from the Five Great Nations, are actually living chakra in physical form, often referred to as "Chakra Monsters". The beasts are differentiated by the number of tails they have, with each possessing an amount from one to ten. They are large, living forms of chakra, giving them power that far outmatches most ninja. However, this immense strength is tempered due to their bestial nature, which restricts them from being smart enough to use it effectively. Beasts sealed within humans are able to benefit from the knowledge of their hosts, giving them the ability to speak and better utilize their powers.
The Land of Mountains' Tailed Beasts have been around before the the founding of Yamagakure 2. Because of their immense power, the shinobi from each clan of the land tried to use the Tailed Beasts as military weapons during the Civil Wars of time's past. The beasts are extremely elusive in the land's written history, they were often regarded as just myth and folklore. Yet despite the status of the beasts as myth, countless shinobi have claimed to encounter them on missions throughout the land. Lately it has been the goal of Yamagakure 2 to capture these beasts for military purposes.
Main article: One-Tailed Beast
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.
Main article: Two-Tailed Demon Sauropod
The Two-Tailed Demon Sauropod is a Land of Mountains Bijū. Two-Tails is a very large, bipedal, dinosaur-like creature. The majority of its body has a light gray, white coloring with linear sections on its body being purple. Its arms and shoulders are covered by large armor plates, and each hand holds five claws. The round plates on its shoulders each have a large pearl-like structure in the center. On it's back are wings, which are seemingly not used for flight as Two-Tails appears to levitate.
Main article: Three-Tailed Demon Horse
The Three-Tails is one of the Land of Mountains Bijū. It is said to appear in conjunction with the arrival of a powerful ninja. It is a good omen that brings rui (Chinese: 瑞; pinyin: ruì; roughly translated as "serenity" or "prosperity"). It is often depicted in legend with what looks like fire all over its body.