@tonysandlin Eric might appreciate that. 
Although he’s only collecting actual Voltorb/Electrode cards and not cards featuring them, I’m sure he still appreciate knowing about the nice link of those two cards. Thanks for sharing, Loch!
Greetz,
Quuador
That’s an awesome trainer card haha I certainly do appreciate this card.
This card has what I always assumed was clouds but is actually a bellsprout. Still not sure if its supposed to be clouds shaped like bellsprout or some weird ghostly image of bellsprout.
@pichufan , awesome discovery and it reminded me of the postcards from the Pokémon Card Trainers Vol. 11. I really like these inter connective artworks. After some research I figured these come from yet an other Premium File.
Still a nice find!Cracker
@cracker Nice post! One missing, which I came across a few days ago are the XY promos for Regigigas, Arceus, and White Kyurem:
I’ve also posted an entire list of related artworks here on reddit a few days back. Here are some related cards that aren’t mentioned in this thread yet below. Most are not really hidden, but still worth mentioning in case someone was unaware of them - most of them tell a story over multiple artworks, sometimes even spread among multiple sets, and some have connected artworks:
Greetz,
Quuador
Okey last update 
first some well known “hidden Pokémon”
Here an updated picture collection
GreetingsCracker
Given the popularity of this card, it’s very suprising to me that this hasn’t been mentioned yet (at least I think so) - or it’s so obvious that it doesn’t even classify as ‘hidden’. Anyhow, I noticed it just a few minutes ago.
And no, I’m not talking about Haunter and Gengar in the back
On the bottom right corner, something is creeping over the railing towards poor Psyduck. I think it’s a Sableye.
One thing I noticed today is that the Rising Rival Rotom RT cards have the appliance of another Rotom in the background, going full circle:
@goldcrest might already know about this one, but if not I’m sure she’ll appreciate the info above. 
Greetz,
Quuador
After watching @smpratte’s latest eBay auction thread video just yet, I noticed the 20th Anniversary Pikachu has a Kakuna in the background and the Mew has a Beedril in the background:
Greetz,
Quuador
Could there even also be a Caterpie in the upper right corner?
I never noticed this and I spent months searching for one. Wow.
I’m digitising my Shared Artwork collection and part of that process involves me going through each of the cards and noting down what Pokémon appear in each artwork.
I decided to overlay the Southern Islands artwork over some of the cards to see if I was missing anything obscure. Turns out I was, but you’d probably have to be an incredibly hardcore collector to even care about these…
Along with Dodrio and Weepinbell, Ledyba’s foot and Butterfree’s wing make a very small appearance on the Jigglypuff card:
On Lapras the Dratini is there in plain view, but part of Magikarp’s fin appears as a tiny white dot in the top left corner:
I had thought Exeggutor and Vileplume were the only two cards in the set which didn’t feature other Pokémon, but uh…
The tip of one of the prongs on Slowking’s crown appears in the bottom right corner of Exeggutor:
Finally - and this one is probably a bit harder to see, I had to add a different filter as the holofoil complicated things - there’s a tiny purple dot next to the top left corner which touches the evolution box on Vileplume, that’s part of Venonat:
I can’t imagine anyone ever thinking “OMG I need to add one of these to my Venonat collection!!!” but hey, technically it’s there.
Doesn’t get much more hidden than this I don’t think!
@quuador I think @pichufan has posted this card enough times but I just noticed a little Pikachu talking to a lucario in the background
Dang, you couldn’t tell? 
it’s pokemon collector from HGSS
@mthursty nice! I’m sure I must have noticed those at some point but I had completely forgotten they were there. This is one of my favourite artworks, here is a blown-up digital version:
Pichu, Smoochum, Pikachu for sure. The Lucario is a bit questionable for me though.
I’d like to also say that behind the chair on the shelf is a fossilized Staryu but it has too many arms - and not enough arms to be a Starmie.
For those precious moments when collectors of specific Pokémon are thrown into a quandary over whether they should care that 1 pixel of their favourite Pokémon appears in the artwork of another card?
That’s why I’m on E4. 
Haha, I’m on E4 for all the random information I didn’t know I wanted to know about This was 10/10
Edit: And for the memes ofc