Pokémon GO has had one noticeable absentee from the generation three games – Kecleon. Indeed, it has been over five years since the first wave of generation three Pokémon released in the app. Today, that has changed – following Chespin Community “Day” (2 to 5 p.m. local time), Kecleon will start appearing on PokéStops (it may take until 5:30 p.m. for it to kick in).
You can see it (or its outline) on distant PokéStops once you click on them (Update: apparently a bug and may be patched by Niantic, so make use of it before that happens!), and seemingly have at least a couple hours before it will despawn. However, you then must get within spin range of the PokéStop to be able to tap the Kecleon a few times to get it off the PokéStop. It will then spawn on the overworld following a message, and you can encounter and catch it. Be wary if you have a Pokémon GO Plus – if it interacts with the PokéStop before you get to the Kecleon, you may lose your chance at the encounter!
Kecleon may be most common for a couple hours after the end of regular Community “Day” hours, and then rarer. It isn’t common to begin with, either. But you only need the one for your Pokédex! It cannot be Shiny as yet either. Good luck getting your Kecleon!