Pokémon Let's Go Pikachu and Eevee don't release until November, and it seems like the next few months up until launch will be filled with reveals for monsters that did not appear in the original Pokémon Yellow. We already knew that Alolan forms of Gen 1 Pokémon will appear in Let's Go, and now thanks to a new trailer and press release, we know that there will also be Mega Evolutions for some Gen 1 creatures too.
As shown off in the new trailer, Let's Go will feature Mega forms for Charizard, Venasaur, and Blastoise. The press release states that these will be "the first three Pokémon that fans will be able to Mega Evolve during their adventure," suggesting that others will also support the form. I'm personally crossing my fingers for Mega Alakazam.
Pokemon let's Go Pikachu and Eevee both release on November 17th 2018.