Specimen X

Specimen X is an alien kaiju created by Toho and a major antagonist in The Dawn of Monster Kid. He is a Spore Monster that attacked Mars and responsible for the extinction of the Martians.

Personality
Specimen X is a mindless creature.

Origin
Specimen X was originally a Spore that was genetically engineered by the Martians. During the experiments, Martian scientists injected their DNA into the Spore, causing it to evolve into a giant monster.