Lava Beast

Storytime! (Pretty bad one)
Before life ever came to Earth (and was as small as a moon), there lived creatures of molten lava that dwelled in molten filled caves & other such terrain. Being not-so-nice guys, they run around burning everything in sight, causing mayhem all around. With nothing left to burn, most died out, becoming part of the outer/inner cores of the Earth, thus growing it to regular size. When life emerged by then, they were all gone, all except one. Living in a cave to sustain itself, it will eventually come out to wreak havok once more.

Eventually...