Tapu Koko casually slapping Kukui's final pokeball must have been insanely depressing for whatever Pokemon was inside there, besides I can't imagine why Kukui wouldn't use Incineroar last since it's his ace
Professor Kukui is in possession of a sixth Pokémon, as its Poké Ball was seen in Fiery Surprises!. However, Tapu Koko slapped its Poké Ball away before it could be sent out, making Kukui's sixth Pokémon unknown.
According to Bulbapedia, it is still unconfirmed.
Source
it wasn't ever revealed so only speculation for us to find out what it was
