True Dragon

The True dragons (Highest tier Divine Monster Spirit) are considered the strongest beings in the world, being equal or superior to a True Demon Lord.

They are beings "perfect as one", especially durable, spiritual life forms, so they have no attachment to their physical bodies, they only take those forms to respond to the desires of religions or similar (or also for convenience)

Birth
Dragons are immortal and unblemished beings made of pure energy and formed by sentient beings’ prayers.

Originals
Star King Dragon Veldanava /Velda (Angra Mainyu): 

The greatest of the True Dragons and the strongest among them. He is also the God that created in the world in which they live. He also is the husband of Lucia and father of the Demon Lord Milim Nava.

Ice Dragon Velzado

Partner of Guy Crimson

Scorch Dragon Velgrynd

Partner of Rudra Nam Ul Nasca

Storm Dragon Verudora/ Veldora Tempest

The youngest of the four True Dragons originals. He is connected with Rimuru by an agreement and was known to rampage here and there

Not official
Milim Nava

She is the daughter of Veldanava and Lucia and it is who inherited the greater part of the power of the soul of its father.

Rimuru Tempest

He became a True Dragon after eater and analyzing the magical power of Veldora in his fight against Velgrynd. He is also a partner and best friend of True Dragon Veldora Tempest.

Trivia

 * In the Light Novel, Milim Nava is considered as a True Dragon and The Youngest of 4 Dragon Kind beside Velzard, Velgrind and Veldora. while in Web novel she doesn't since Rimuru is the 5th True Dragon instead the 6th True Dragon.