Gobi no Irukauma, takes the form of a five-tailed dolphin-horse. Son Goku is also known as King of the Sage Monkeys. Yonbi no Saru, takes the form of a four-tailed monkey. Sanbi no Kyodaigame, takes the form of a three-tailed turtle.
Nibi no Nekomata, takes the form of a two-tailed cat. Ichibi no Shukaku, takes the form of a racoon. As the Sage was close to death, he used his "Creation of All Things" ability to split the monster into nine tailed beasts and scattered them across the world.
However, this method was only a temporary way of sealing the demon, as once the Sage died, the Ten-Tails would be free again. The Ten-Tails was subdued by the Sage of the Six Paths and sealed within his body to become the first Jinchuriki. The nine separate tailed beasts that exist today were created from the chakra of the the Ten-Tails, the original demon. Every demon has a different amount of tails, ranging from one tail to ten. When the human has a demon sealed inside them they are known as Jinchuriki. The Fourth Hokage was one of those people who sacrificed their life to seal a Bijuu and protect the world from chaos.
To try and combat these powerful beasts and stop the destruction they cause, they were captured and sealed inside new born babies using a powerful technique that would kill the person using it. Each one has an incredible amount of chakra within them. Tailed Beasts (Bijuu) are demons that exist in the world of Naruto, they are also known in as demons in Japanese mythology.