Every item that is a drop from a creature, whether it is a boss or not, is listed as creature product, if its not a part of a set, a weapon, ammo or decoration (dolls are an example for this last cathegory) then it means its a creature product and you will find them listed in that cathegory.
EDIT: Forgot to add the most important part of this answer LOL: There is other items listed in Creature products cathegory that fit into helmets (Broken Visors, for example) or other types of items, but they only get listed for the most important characteristic of the item itself. This means, while Ferumbra's Hat has some armor, it is most commonly use as a creature product than anything else.