Rex did not originate from fanon, and more information can be found on their respective wiki(s). You may read more about it on the following wiki(s):

Rex by Bowserthesecond-
A Rex.
Species Origin Dinosaur/Dragon
Alignment Neutral

Rexes are blue dragon-like dinosaurs that first appear in Super Mario World, later making more minor appearances in games like Super Princess Peach and Mario & Luigi: Superstar Saga. They live on Dinosaur Land and serve as enemies against Mario. They move along the ground, similar to Goombas and Koopa Troopas. However, when Mario stomps on them, they will become flattened and begin moving at double speed. A second stomp would defeat it permanently. Alternatively, a spin jump, Fireballs from Fire Mario, a spin from Cape Mario, and Invincible Mario can defeat them in one hit.


Super Mario Dino Riders=

Rex appears in Super Mario Dino Riders as a rideable mount, his first special is Stomp.

The Marvelous Misadventures of Bowser

Rex appears in The Marvelous Misadventures of Bowser.

Super Koopa Kart

Rex appears as a medium weight driver in Super Koopa Kart.

New Super Mario World (3DS)

They reappear in this game. This time, there will be two new sub species: Flying Rex, which will fly, and Pyro Rex, wich will shoot fireballs. In this game, we will discover that Draggadon is somehow related to Rex.


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