Kaiju Mario (怪獣マリオ, Monster Mario) is a monster doodle of Mario created by Yoshi on a rigged version of Mario Paint that appears as the main antagonist of chapter 10 in Super Mario-kun volume 4. Yoshi drew it "like he sees Mario": an angry beast, complete with a reptile tail, fire breath, sharp fangs, and a pointy mustache.
After Mario asks Yoshi to draw him, Yoshi's first doodle comes out badly, so Mario tells him to create another one, suggesting maybe to concentrate more on how he sees Mario. Yoshi creates Kaiju Mario, but Mario yells at him to get rid of it, but then Yoshi notices another icon near the Undodog's: a Bowser icon. Yoshi presses it and Kaiju Mario comes out from the screen and attacks them. Yoshi, drawing a watermelon and pressing the Bowser icon again, realizes how it works: the drawing comes to life and attacks the artist.
Kaiju Mario attacks the heroes left and right by stomping them, slapping them with its tail, singing deaf-tone tunes, vomiting paint on them, and burning them with his breath. Yoshi tries to melt it with water, but fails, so the heroes try to reach for the SNES Mouse and undo the creations, but are constantly stopped by the monster. Finally, Yoshi throws a Super Mushroom at the SNES Mouse, summoning the Mario Paint's eraser. With that, Mario destroys Kaiju Mario, slicing him into confetti-size pieces.