Ray Stantz had no idea what he was getting his fellow Ghostbusters into when he accidentally summoned this jolly-looking fiend. In his defense, a giant walking marshmallow doesn't sound like much of a threat to New York City, but when that campfire treat ends up being brought to life by a demonic god, you'd better put the chocolate and graham crackers away and reach for your proton pack.
Of all the skeletons, spirits, and haunts that end up terrorizing the Big Apple across both Ghostbuster films, the Stay Puft Marshmallow Man might well be the most iconic of the bunch, if only for his involvement in the climax of the first movie. He gives Slimer some stiff competition, anyways.
At the very least, he's one of the few "name" ghosts that lasts beyond the lifespan of the movies, coming up again and again in the TV show at first as a villain and then as a helpful ally to the team later on in the series. He's still pretty adverse to sitting by an open flame, though.