Well Legend speaks of a secret civilization hidden in these mountains. It speaks of how this civilization revered the Juubi and were at one time it's minions. Because the Juubi was unable to carry out menial tasks it corrupted a nearby civilization in order for them to carry out it's bidding. This civilization were made out of Ogres and their dogs, the Ogres were dumb and easily corrupted, and the dogs did only their masters' bidding. For many years this civilization prospered and helped the Juubi spread its corruption and malice.
One day the Sage of the Six paths came to vanquish the Juubi and free the world from it's unholy grip. The Juubi fought the Sage but ultimately the Sage proved to be the stronger one and so the Juubi called out with it's mind, it called it's minions to assist it. But the its minions had seen the power of the Sage and were too afraid to fight. When the Juubi realised this betrayal, it sent some of it's remaining chakra and cursed it's minions for their cowardice. He cursed them to be immortal and undying to forever roam these mountains and never seen again. It is unclear to what they actually look like now as the corruption from the Juubi took it's toll on their bodies and minds.
It is said they still roam these mountains to this very day, waiting to be freed from their shackles of unending torment.