OnceUponATime
Wicked
okay this needs to be updated because Nihilego appeared in today's episode, and while it was only in a Flashback I do think that still counts as a debut.
And also Silvally didn't appear at all in the episode, and instead it seems like Gladion calls his Type:Null Silvally, even though it hasn't "evolved" yet.
Got it, it's just been a busy day. The list has now been updated.