Have there been any times where Pokemon were revealed in the expo demos?

[TapLover]

Taken Yamper and Impidimp for example. They weren't revealed in the June Pokemon Direct, nor are on the official site so we are unsure of their personality and habitats. Both were first seen at E3, where the demo was first played. Have there been any other similar examples?

 

[Esserise]

Yamper is on the official site, and was formally revealed in July's Gigantamax trailer. It is also prominently featured in the illustration of the camping feature.

I don't think there's been an exact 1:1 instance of what's going on with Impidimp before, but there have been cases in which new Pokémon (such as Ribombee) were only glimpsed in some piece of promotional material and were not formally revealed until much later.

 

[Nicolas721]

In Gen V it happened a lot, Pokémon randomly appeared in footage and their existence wasn't confirmed until much later:
-Drillbur and Gigalith (video footage, confirmed 1.5 months later)
-Excadrill, Cofagrigus, Galvantula and Haxorus (video footage, confirmed 2.5 months later)
-Pansage (merchandise leak, confirmed 1,5 months later)
-Tranquill (appeared in the demo, confirmed until the release 2.5 months later)
-Darumaka (appeared in the demo, confirmed until the release 1.5 months later)

In Gen VI, both Mega Kangaskhan and Trevenant weren't seen until release, both were leaked in official footage.

In Gen VII, almost everything appeared in videos and the ones who didn't were confirmed a month later.

Source

 

[prog rocker]

In Gen V it happened a lot, Pokémon randomly appeared in footage and their existence wasn't confirmed until much later:
-Drillbur and Gigalith (video footage, confirmed 1.5 months later)
-Excadrill, Cofagrigus, Galvantula and Haxorus (video footage, confirmed 2.5 months later)
-Pansage (merchandise leak, confirmed 1,5 months later)
-Tranquill (appeared in the demo, confirmed until the release 2.5 months later)
-Darumaka (appeared in the demo, confirmed until the release 1.5 months later)

In Gen VI, both Mega Kangaskhan and Trevenant weren't seen until release, both were leaked in official footage.

In Gen VII, almost everything appeared in videos and the ones who didn't were confirmed a month later.

Source

Don't forget that Audino was included in the demo, too.

EDIT: Also, Gothitelle, Excadrill and Reuniclus were seen in the Triple Battle reveal feature and went unrevealed for a time. Ribombee was seen in one of the "Train On" videos before being revealed