Kyuu-Tales
織田信長☆FAN
The legendary in question could be like Magearna: the first Pokemon reveal for Generation VIII.I think it was hinted that a new Mythical would be in the next movie.
Indeed, I noticed the cover of the guide seems to comprise most if not all of Melemele's Dex as well, in addition to part of Akala's. We still don't know about Ula'ula's or Poni's, though.