I think that one thing that might make the Ice-type Slugma fake, at least according to me, is the fact that Slugma is the Lava Pokémon. With the exception of Hoopa, Pokémon that have different forms keep the same Pokédex category, so would it make sense for an Ice-type Slugma to be called the Lava Pokémon. Another bit of information that I also came up with might provide evidence for the speculated third Lycanroc. Rockruff has 3 abilities, Keen Eye, Vital Spirit and Steadfast. When Rockruff evolves into Midday Lycanroc, Vital Spirit is replaced with Sand Rush. When Rockruff evolves into Midnight Lycanroc, Steadfast is replaced with No Guard. I believe that this hypothetical 3rd Lycanroc will have Keen Eye replaced.