I'd argue the difference between a IP, an Original, and a IP-Original thrives on different aspects.
IP = An IP that properly takes itself as a chance to adapt the IP truthfully, to make an adaptation of material that while can have unique elements; must stick to a specific set of what's expected (Examples: Us, The Weeknd: After Hours Nightmare, Halloween)
IP-Original = The Use of an IP that has the chance to diversify and explore into an IP to see what would constitute it to have a scenario made for it in mind of the event (Examples: Texas LOL, The Weeknd: Nightmare Trilogy, the UM HHN series [as of 2018], Titans of Terror, and Chucky: Ultimate Kill Count)
Original = The Use of original concepts and themes, using iconography and imagery that comes from non-IP based content. Including Orlando Versions of concepts, which have diversified off of what made those concepts unique; to curate stories impactful for USH's event (Examples: Scarecrow, Dead Exposure, the Monstruos series, La Llorona & El Cucuy and Exterminatorz).
Giving my two cents, I believe Terrifier is going to be the Original-IP for a levy of reasons. The biggest out of two of them; is that I believe they will want to make a house experience that can be seen for mostly everyone. One that could be as intense and scary as say, Texas LOL, but one that uses the simplicity that is Art The Clown and to have him work for Universal.
Also think it in theory makes sense to rely on "Art The Clown" as a character of importance, that can be a key icon in relevancy and iconography for the event. The merchandising and the synergy opportunities that you have with the character makes me think USH is going to take a rare opportunity and "Jigsaw/Freddy" him.