Alongside the release of a Pokémon Sun & Moon, a toy-version of the Rotom Dex which allowed players to view 100 new Pokémon that were added in Generation VII was also sold in Japan.
Now, to correspond with the release of Pokémon Ultra Sun & Ultra Moon, an upgraded version is being advertised with a bigger screen and “complete records of Alola’s 400 Pokémon”! That’s an interesting number, considering the original Alola Pokédex capped out at 302 (the last Pokémon being Marshadow). Considering it is able to communicate with USUM, that selection of 400 Pokémon seems like a deliberate choice to correspond with the games. It also clearly shows Necrozma being #400 on the Rotom Dex’s screen.
This suggests that 100 new Pokémon which were not able to be found in Alola the first time around will now be able to be captured in the region!
If so, this makes it quite a large addition compared to past same generation follow-ups. Platinum expanded Diamond & Pearl‘s Pokédex by about 60, while Black 2 & White 2 added around 150 to their predecessor’s (although in the original Black & White featured solely new Pokémon, unlike other games, which makes BW2 seem like a bigger jump).