The Big Rock Candy Monster (formerly known as the Big Monster) was the main antagonist of the episode Marshmalia. He was jealous of the Marshmallows because a long time ago, the main ingredient of s'mores was rock candy until it was replaced by marshmallows.
He then came threatening to destroy Marshmalia. King Marshmallow sent his troops to destroy the Big Rock Candy Monster, but their attacks were no use. Orange and the gang made an attempt to melt him with their lasers, but the attempt failed, as his skin formed a reflective surface.
They then lured him into Caramel Lake, whereas they boiled the lake to a consistent temperature, reducing the Big Rock Candy Monster into a liquid substance. He has made a few cameos since then, but returned for a more major appearance in Marshmallow Wedding, informing Marshmallow about a surprise engagement with Grumblabla.
He had also claimed that being boiled in Caramel Lake taught him a lesson, and since then he had promised to leave his evil ways behind and become a hero instead. It was revealed in Mash of the Titans that he used to be a reptalius monster before staring into Medrootsa's back roots, which turn any living thing into solid rock candy if stared at.
After this, the Big Rock Candy Monster (who was at the time known simply as the Big Monster), claimed that he would go to a planet with bunnies, rainbows and a kitten train. With that, he used his engine-like abilities to fly to the planet of his desires, known as Marshmalia.