ZIZ 03

The protectors of earth killed, their souls captured and used to power the engines for these machines. There is no longer anyone left to save the planet. It’s up to a ragtag group of heroes to find each monster, climb inside, and calm the crying soul of the Dragon God within it.

These creatures are MASSIVE, Ziz alone is larger than most houses. As the code number of the machine gets lower, the actual size of it grows larger. Ziz can ride Behemoth, both can fit on top of Leviathan, and all 3 fit comfortably inside of Bahamut.

Ziz is a fighter jet that can fly at blinding speeds Behemoth is a tank that also has the ability to burrow underground Leviathan is a war ship but can also fully submerge under water Bahamut is a space station that can fire massive, planet destroying lasers

There's more info on the history of these characters in this twitter thread