Rex is a dragon-like creature that's the size of a pony with wings that are wide as a garage door. He has bluish-violet skin with small scales on his back.
Rex acts like a friendly dog to Lucas a while after the latter took care of the former's injuries. But he acts like a carnivorous wolf around Nathan for thinking about killing him.
A bunch of dragon creatures were created from Mighty Mountain, a Monster Empire lab hidden in the mountains. Some scientists were trying to force the beings to obey them with torture, but the dragons became furious and broke out. The dragon creatures had escaped, but one of them was injured by a sudden explosion.
Mighty Mountain Arc
The dragon creature's injures from escaping the Mighty Mountain finally forced him to fall into the floating Rebel City. He would've died if Lucas had not saved him. The creature had to be hidden from Nathan Legend and Daphne Skysight fearing that the creature would be killed by them. He was soon found out and then studied by Flint and was shown to have been a creature created by the Monster Empire.