Mega Pidgeot in Pokemon GO was promised as a reward for the completion of 2 million mega raids during the first week of Mega September, although there has been no word yet from Niantic on whether or not players actually completed the challenge.
If it is revealed that Mega Pidgeot has been added to the game, it's likely that the Pokemon will Mega Evolve the same way all of the other Mega Evolution capable Pokemon do in GO.
Mega Pidgeot Pokemon GO: How to Get
Once a Pidgeot has been acquired by fully evolving a Pidgey, which requires a total of 62 candies, one can Mega Evolve their Pidgeot by collecting Mega Pidgeot Energy.
Mega energy can be collected by battling in Mega raids or completing special field research tasks. It's fairly likely that Mega Pidgeot will have a similar initial Mega Energy cost to Beedrill, which initially needs 100 Mega Beedril energy to Mega Evolve.
As a Pokemon Mega Evolves more often the cost to do so progressively decreases. In the case of Beedril for example, after the initial 100 Energy cost Mega Evolution it only requires 20 Energy to Mega Evolve. More powerful Pokemon like the Kanto Starters require a higher amount of energy for initial mega evolution.