[Chapter will always contain spoilers up till the latest updates. Proceed at your own risk.]
These are the mythological creatures that have been shown in the story so far. The direct explanation of these creatures does not exist yet. Even the question of them actually existing in the world or not has not been answered yet. However, special type of mages called Tamers can use cards to summon them to do their bidding.
[This chapter will be updated whenever some new information about this is found.]
1. Sphinx
A sphinx is a mythical creature with the head of a human and the body of a lion. (though this appearance is not fixed and varies depending upon various iterations of various mythologies)
It has showed up in Persian, Egyptian as well as Greek mythologies, though its origin is said to be mostly Egyptian. The Great Pyramid of Giza has a statue of this creature called "Great Sphinx of Giza" in front of it.
The card to summon this creature has been shown to be possessed by Danny so far.
Visit the Wikipedia page for more information -> https://en.wikipedia.org/wiki/Sphinx
2. Cerberus
In Greek mythology, Cerberus, often referred to as the hound of Hades, is a multi-headed dog that guards the gates of the Underworld to prevent the dead from leaving. It is mostly known in mythologies for the fact that Hercules' 12th and final labor was to capture and bring it back from Hades, the king of the underworld.
The card to summon this creature has been shown to be possessed by Danny so far.
Visit the Wikipedia page for more information -> https://en.wikipedia.org/wiki/Cerberus
3. Manticore
It has the head of a human, the body of a lion and a tail of venomous spines similar to porcupine quills, while other depictions have it with the tail of a scorpion. (In this story, his tail is like that of a scorpion). It also has wings like that of a bat. It's lion-like appearance has often made it a topic of comparison with Sphinx.
It's origin is considered to be Iran, and it has appeared in Persian mythology for the most part.
It's notably considered one of the fastest mythological creatures.
The card to summon this creature has been shown to be possessed by Grey so far.
Visit the Wikipedia page for more information -> https://en.wikipedia.org/wiki/Manticore
4. Raijū
It is a legendary creature from Japanese Mythology and is considered to be the companion of the Raijin, the Shinto god of lightning.
Its body is composed of lightning and with the form of a white and blue wolf, or even a wolf wrapped in lightning, being the most common, although it can be represented with other forms such as tanukis, kitsunee, weasels or cats. It may also fly about as a ball of lightning. Its cry sounds like thunder. (In this story, it has been portrayed as a form of a white and blue wolf)
The card to summon this creature has been shown to be possessed by Jessica so far.
Visit the Wikipedia page for more information -> https://en.wikipedia.org/wiki/Raij%C5%AB