https://www.gamespot.com/articles/last-chance-to-get-a-free-pokemon-in-ultra-sun-and/1100-6455880/

The first distribution for Pokemon Ultra Sun and Ultra Moon is coming to an end soon. Nintendo is giving away a special Rockruff for the new titles until January 10, making this your last opportunity to add the Rock-type puppy Pokemon to your team.

What makes this particular Rockruff different from those you could acquire in-game is that it comes with the Own Tempo Ability, which allows it to evolve into the new Dusk Form Lycanroc. Rockruff you encounter in the wild in Ultra Sun and Ultra Moon do not typically have this Ability, making this your only way to obtain a Dusk Form Lycanroc within the games.