https://www.gamespot.com/articles/last-chance-free-legendary-available-for-ultra-sun/1100-6461626/

Latios and Latias aren't the only Legendary Pokemon players can pick up for free this month. To celebrate the release of the newest Pokemon TCG expansion, Dragon Majesty, The Pokemon Company has announced it is giving away a free Poipole in Ultra Sun and Ultra Moon--but as usual, the way to claim it will vary by region. [Update: The Shiny Poipole distribution is scheduled to end this week, on October 7, making this your last chance to claim a download code for one if you still haven't.]

Originally unveiled under the codename UB Adhesive, Poipole is one of the new Ultra Beasts introduced in Ultra Sun and Ultra Moon, meaning you'll need to own one of those two versions to receive it. What makes this particular Poipole special is that it's a Shiny; unlike most of the ones you're likely to encounter in the game, this one is white and gold rather than the usual purple.