As you can see firsthand, here are the assets Chrales found in the network traffic (obviously placeholder, link to tweet, link to other tweet):
Originally he tweeted that they were referred to as “new kecleon icons.” However, this is clearly nothing related to Kecleon. Kecleon’s eyes look nothing like that, nothing on Kecleon looks remotely hexagonal, and finally it is odd that there is a quasi-ditto underneath (although a comparison between it and a ditto sprite show some differences, it is indeed different than a ditto).
To top it all off, It came with what appears to be a normal, and a shiny sprite, something that would be odd for icons for Kecleon. Chrales later clarified in this tweet, stating that :
some more info about the kecleon “error”
it’s labeled as kecleon in pokémongo but unity files are tagged with pokémon 891
— Chrales (@Chrales) September 21, 2018
For those keeping track, the current national pokedex up to Ultra Sun and Moon sits at 807, ending with the recent Mythical Zeraora. And as you can see, it is labeled with a tag of 891, 84 entries above the known maximum. This coupled with the fact that in the Let’s Go announcements, it was stated that Let’s Go would only feature 1 new Pokemon, that can also carry over to Pokemon GO, and that has also never been seen in any game up to the recent Ultra Sun and Moon Generation 7 games (links for that here and here) well, we would consider this a open and shut leak.
Even more news! Trainers, the special Pokémon referenced at the end of the #PokemonLetsGo trailer is referring to a Pokémon never seen before! pic.twitter.com/Vm8uCAhHSJ
— Pokémon (@Pokemon) May 30, 2018
We do try to hesitate with anything unconfirmed, however the fact that a asset came through with a shiny asset to boot, along with the knowledge that Let’s Go and Pokemon Go are intimately tied together, seems to show that Niantic put out something earlier than they should have.