I was wondering whether there is a card that can make a creature a horror in addition to its other types. The card type does not matter. Is there a card like this?
Yes: Xenograft is an enchantment that adds a creature type of your choice to all your creatures. There are also many cards that add all creature types to one or more creatures, including the creature cards Amoeboid Changeling and Mirror Entity, and the instants Blades of Velis Vel and Shields of Velis Vel.