*whispers* Some say that Rarity resembles a marshmallow. Common joke, but she gets quite mad about it. Sometimes violent ... *looks around* ... No words were spoken here today! What a beautiful weather we have! XD
Not 100% certain, but I think it came from the Dog and Pony Show episode, where there's a shot where Rarity has her front legs extended and they look exactly like elongated marshmallows. Somebody made a macro of that shot with the words "no one must know I am secretly part marshmallow" and I'm pretty sure that's what started it.