In almost every situation, once a Shiny Pokémon can be encountered in Pokémon GO, it will be available in its Shiny form in any type of encounter. Now that Mega Ampharos has been introduced into raids with a chance of a Shiny encounter after the raid is defeated, you know what that means: wild spawns of Ampharos can now be Shiny as well!
For those that didn't know about Mega Raids unlocking Shiny forms in encounters outside of Mega Raids, it is, indeed, true. The evolved species that can now be encountered in their Shiny forms in the wild thanks to Mega Raids include:
Now, while these encounters outside of Mega Raids in Pokémon GO are indeed rare, they do exist. The Season of Celebration has made it so that, outside of events, evolved forms are more common encounters than before. As an anecdotal example, I generally see between one and three Ampharos in the wild or responding to Incense every week since this current Season launched in December. During the holiday event, Ambomasnow was a fairly common spawn.
This new way to encounter evolved Shinies in Pokémon GO has been a positive impact from Mega Raids, a mechanic that is otherwise not exactly beloved in the game's community. Personally, as Pokémon GO flounders a bit with their events, raid rotations, and Community Days in the past two months, I personally believe that the game has way bigger problems than Mega Evolution and Mega Raids, mechanics that were essentially fixed by Niantic as a result of community feedback. Hopefully, Niantic can mirror their reaction to Mega Raids with the current in-game problems and adjust based on the overwhelming community feedback at the current state of the game.
In any case, encountering a wild Ampharos, which was already spicy indeed, has now become even more exciting. Good luck out there, fellow trainers.