The tailed beasts (尾獣, bijū) are the nine titanic behemoths within the Naruto Dark Clouds series. They are differentiated by the number of tails they have, ranging from one to nine. The tailed beasts are living forms of chakra, sometimes referred to as "Chakra Monsters" (チャクラのバケモノ, Chakura no Bakemono), giving them immense reserves of especially strong chakra that far outmatches most shinobi. The tailed beasts' chakra is coloured red.
They also have the ability to communicate telepathically with each other and their jinchūriki, and possess several levels of consciousness. There is the initial level where the beast may appear in a habitat of sorts seemingly imprisoned in some way by whatever fūinjutsu that sealed them. The second level is where all the tailed beasts can convene, free of any restrictions and communicate with one another. The jinchūriki are also able to enter there, but in order to do so, they must be completely linked with their tailed beasts.
The tailed beasts are also able to freely transfer their chakra to whomever they choose, and as Kurama has noted, is a less complicated process for them than it is for humans.
Long ago before the founding of the hidden villages, during an era of endless wars between mankind and the other races, there existed the Shinju, a tree that was revered as a deity which bore a fruit once every thousand years. Although it was forbidden to consume the Shinju's fruit, it was eaten by Princess Kaguya Ōtsutsuki, who used the power it granted her to end the wars plaguing the lands. Some time after this, Kaguya gave birth to two sons, Hagoromo and Hamura, who became the first humans born with chakra. With their powers the Ōtsutsuki brothers managed to end the wars and bring a state of piece to the world. However, their strongest fight would come to be against the demon Ten-Tails, created by their mother, who was enraged that her power had been spread to her children, by using her Creation of All Things ability to turn the Shinju into a monster.
The Ten-Tails rampaged throughout the lands until Kaguya's sons managed to defeat the beast and seal it's chackra within Hagoromo's youngest son, Asura, who became the first jinchūriki in history. However, the prize was high, Hamura had to sacrifice himself for them to seal such power. However, knowing that should the demon be unleashed back into the world, he would continue its rampage and destroy everything for which his brother sacrificed his life for, the Sage used his Chibaku Tensei to seal the monster's husk in what would become the moon. This act, along with many other great deeds throughout his lifetime, resulted with Kaguya's son to become the "God of the Six Paths", becoming the first male human to ever become a God.
As for Hagoromo's youngest, Asura, he would later use Living Magic and Arc of Embodiment to divide the beast's chakra sealed inside him into the nine tailed beasts and gave each a name. Some time after their creation, the young tailed beasts were told that they were all still linked to one another despite being separate entities. Asura also told them that they would eventually become one again, yet not as they originally were, and that one shall appear at that time to show them what true power is. Despite their close relation to one another, there is a significant amount of friction between the tailed beasts that caused them to part ways. One such cause was due to Kurama's belief that their strength was determined by the number of their tails.
Over the centuries, countless humans failed to regard the tailed beasts as sapient individuals, treating them with great disdain and fear. Because of their immense power, they sought the tailed beasts out as tools to be controlled, a sentiment they detest. By the time of the formation of the hidden villages, after his victory against Madara Uchiha in the Valley of the End, Hashirama Senju, the First Hokage, used his Wood Release techniques to capture several of the tailed beasts and allow the other hidden villages to purchase them in exchange for signing peace treaties with each other, in order to stabilise the balance of power between them.
Madara, later revealed to have survived the altercation, had obtained Hashirama's DNA — his true purpose for the encounter. Integrating Hashirama's DNA into his wounds, Madara awakened the Rinnegan shortly before the end of his natural lifespan. With it, he was able to summon the Ten-Tails' husk from the moon and, now, he's trying to capture the tailed beasts, with the help of the Ministry of Darkness.