What are the Tailed Beasts (Bijuu) actually made of?

Question

What kind of creatures are these Tailed Beasts?

I know they are the summation of huge amount of chakra and a body.

If they are just chakra, can't the chakra itself be the body?

What do Tailed Beasts need the body for?

Answer 1

Bijuu are living beings, same as humans and animals. They were created by the Sage of Six Paths, using the Creation of All Things technique.

The sage created the vessels with Yin, breathed life into them with Yang, then poured the Ten Tails' chakra into them.

Chakra is merely energy. It cannot take complex physical form (With the exception of the Creation of All Things, which uses chakra to create form).

Answer 2

As per what has been revealed.

Juubi (Shinju) was initially a Tree before taking the beast form.
This tree produced one single fruit every 1000 years. And it was considered as a forbidden fruit and should not be consumed by anyone.
However, this law was broken by a princess name Kaguya Ōtsutsuki and she gained unbelievable amount of power, which she used to end all the wars and clear lands.
Then, Kaguya gave birth to the first human with chakra. He was called Hagoromo.
Then came the awakening of the Ten Tails due to the mistake made by Kaguya.
The Ten Tails went on rampage and started destroying everything.
Hagoromo somehow managed to defeat it and sealed it within himself. This made him the Sage of Six Paths.
Hagoromo separated the monster's chakra from its body before using Chibaku Tensei to seal the husk in what would become the moon.
Then he used his Creation of All Things ability to divide the chakra into the nine Tailed Beasts.

So in short, this is basically chakra which is stored in a vessel. Here the Tailed Beasts are just vessels. You can even consider it as 9 different beasts which were originally normal animals, but they got their powers when the Sage gave them chakra. That can be a reason why every Tailed Beast resembles a particular animal.