With Mega Evolutions now available in Pokémon Go, Niantic has also added Mega Raids to provide players a chance to capture Mega-capable Pokémon and earn more Mega Energy to use the new mechanic.
As of now, you can find Mega Blastoise, Mega Charizard X, Mega Charizard Y, and Mega Venusaur in Mega Raids and all of them can be encountered in their Shiny forms.
Whenever you complete a Mega Raid, you will earn Mega Energy based on how quickly you beat the opposing Pokémon. In a normal raid, that bonus would become extra Premier Balls that would give you extra capture attempts against the raid boss.
Once you do defeat a Mega Evolved raid boss, you will have an opportunity to capture its normal form. Each of those Pokémon has a chance to be Shiny, but it is a very slim chance to see the color variant and special animation.
Because these are the first Mega Raids, Niantic is likely to include Shiny forms in most if not all Mega Raids moving forward.
Mega Beedrill is also available in the game but not through Mega Raids. You can still train up a Shiny Beedrill and make it Mega Evolve once you get enough Beedrill Mega Energy from various research tasks and other sources.