When you see horns, crowns, or other such things on someone's head (that are not cloak-hats), it's not an item worn in a slot, it's a visual effect with a model that looks solid and is positioned to appear to be on the character's head. There has to be one visual effect per race, too, because the head position is different for Humans, Elves, Halflings, Gnomes, and Half-Orcs, but that is how that stuff works.
Theoretically you can stick effects that have models anywhere on any character's body, provided you've created the visual effects for it.