It looks like the popular Mega Evolutions introduced in Pokemon X and Y are making their way to Pokemon: Let’s Go, Pikachu! and Pokemon: Let’s Go, Eevee!
This news comes from Pokemon fan site Serebii, which scanned pages from the latest issue of the Japanese gaming magazine CoroCoro. The scans show off Mega Evolutions for the final evolutions of all three first generation starter Pokemon: Mega Charizard (both the X and Y variants), Mega Blastoise and Mega Venusaur.
It appears that the way players will Mega Evolve their Pokemon is similar to past games. In the past, Pokemon needed to hold their respective Mega Stones in order for their Mega Evolution to occur. Only one Mega Evolution could happen for each trainer during any given battle. These Mega Evolutions significantly raised the Pokemon’s stats, and often temporarily changed their typing.
Source: IGN Video Games All