A number of image assets were just added to Pokemon Go, indicating which Pokemon could be the first to get their Mega Evolutions. Yesterday, dataminers discovered image assets for the Mega Evolved versions of Venusaur, Charizard, Blastoise, Beedrill, and Pidgeot were added to Pokemon Go, indicating that Mega Evolution could be coming very soon. A Mega Raid Egg asset was also added, seemingly confirming that the new mechanic will tie into Raid Battles, with players collecting "Mega Energy" from Mega Raids and then using them to temporarily Mega Evolve their Pokemon.
Mega Evolution is a relatively recent mechanic to the Pokemon franchise, having first been introduced in Pokemon X & Y. In the main series games, Pokemon are equipped with special Mega Evolution stones that give them the power to temporarily transform into stronger Pokemon. Most Mega Evolutions gain stat boosts of some kind, and often have different abilities than their base form. While some aspects of Mega Evolution are still unclear for Pokemon Go, we do know that this will be a temporary change, likely tied to "Mega Energy" that a Pokemon consumes over time.
The fact that only five Mega Evolved forms were added to Pokemon Go seems to indicate that Niantic will be following its "slow drip" method of releasing content, focusing on a handful of Pokemon at a time to ensure that players stay invested in the new mechanic. This also allows Pokemon Go to make tweaks to the new mechanic without radically shaking up the entire game.
Pokemon Go has promised an announcement on Mega Evolution soon, so we could see it added to the game as soon as this weekend. Stay tuned for more updates about Mega Evolution in Pokemon Go as we get them.