It's been kept as a huge secret but Gamefreak is not only aware of the Glameow rarity meme, but is in on the joke.

Added in during the last few days of development, Gamefreak has added a secret encounter with Glameow somewhere in the Crown Tundra.

You will encounter one and you can either defeat it or catch it. It will respawn occasionally like the Wailord in Isle of Armor, so don't panic if you don't catch it right away.

It'll have a brief joke introduction about how rare it is before the encounter.