With sixth generation Pokemon making their way into Pokemon Go, it was only a matter of time until Mega Evolutions (which also debuted in X and Y) were added too. Starting with an update tomorrow, players will be able to Mega evolve a handful of Pokemon, but don't expect the process to work like it does in Game Freak's titles.
To start, the only four first generation Pokemon are getting Mega Evolutions in Pokemon Go: Charizard, Blastoise, Venusaur, and Beedrill. Also, it looks like you're only able to get Mega Charizard X at the moment and not its Y-evolution counterpart.
As our sister site Eurogamer explains, Mega Evolutions in Pokemon Go aren't permanent, and you'll be able to see how much time your Mega has left on its CP screen. Additionally, you can only have one Mega-evolved Pokemon at a time-if you evolve a second one, the first will revert back to its normal state. Players also can't do Mega evolutions with Clone or Shadow Pokemon.
While you will be able to use Mega Pokemon in most circumstances (they'll even give other attackers a boost in raids), you won't be able to use Mega Evolutions to defend gyms or in the Battle League. The Mega Pokemon might be a bit too mighty for Pokemon Go's competitive balance at the moment.
Raiding is something you'll be doing a lot of if you want to try out Go's Mega Evolutions, as completing raids against Mega Evolutions is one way to collect the Mega Energy you need to evolve your Pokemon. At least with the advent of Remote Raids, you'll be able to check this stuff out from the comfort of your own home.
So, if you want to reap the maximum benefits of these beefed-up Pokemon, you'll either need to play a lot to keep a steady stream of Mega Energy flowing or be pretty careful about when you deploy them. Coming up soon, though, there will be a Mega Raid-focused event in Pokemon Go running for a week starting on September 1, so it should presumably be pretty easy to collect some or all four of these beginner Megas with some dedicated raiding.