Shiny Cyndaquil Pokémon GO is a rare version of Cyndaquil you can catch if you're lucky. Here's how to find a Shiny Cyndaquil.
Shiny Cyndaquil Pokémon GO: How to Catch
There is about a one in 450 chance of encountering a Shiny Pokémon, with that rate increasing during certain events like Community Day. Events that increase spawns of certain Pokémon help in that you see more of that Pokémon, but the chances of finding a Shiny don't improve.
That being said, the currently ongoing Throwback Challenge is in its Johto leg, with Pokémon from that region being featured and appearing in the wild more frequently. Cyndaquil is among those, so if you're hunting for one now is as good a time as any.
Shiny Pokémon don't appear on the map as Shiny; you will have to engage them to see if they're Shiny or not. Shiny Cyndaquil has a brown back instead of blue.
Why go through the trouble of finding a Shiny Cyndaquil? If you evolve it into its final form, Typhlosion, you'll get a strong Fire type attacker that you can show off as a shiny.
Typhlosion has good type coverage, learning Solarbeam and Shadowclaw. It's similar to Charizard in stats, except Typhlosion doesn't have the Flying type. Its Community Day move Blast Burn puts it ahead, but you'll need an Elite TM to teach it if you missed out.