The Dragon Lord
Yanmega was a pokemon introduced in Gen IV, but in the Johto HGSSdex is #102, not after Celebi .
The relative order being kept was referring to the initial regional dex of that generation's region. Remakes and sometimes third versions play much more loosely with that.
Meltan is the the first pokemon to be obtainable in a spinoff game, GO. It's not a cameo like bonsly in XD.
But it's also obtainable in a main series game, so it doesn't matter.
Still thinking the wikipedia classification as "new pokemon" outside gens is the best.
I won't disagree entirely with Meltan being classified as no generation. My main argument is that it's not Gen VIII, not necessarily that it is Gen VII.