Marsh King is a brand of marshmallows that appears in the episode "The Camping Episode."
The package is transparent with red text that reads the product's name and a golden crown on top of said text.
Role in episode
SpongeBob and Patrick are pretending to be astronauts when wearing water helmets while eating these marshmallows. SpongeBob then offers some to Squidward, but the latter turns down the offer on grounds that he does not eat junk food. However, due to him being annoyed by their astronaut roleplay, Squidward then agrees to have one and tries to roast many marshmallows, but keeps getting spit "molten foodstuffs" at him by Patrick.
Later, when Squidward is playing his clarinet, SpongeBob shoots a marshmallow through the clarinet into his throat to prevent the music from attracting a sea bear, force-feeding it to him in the process.