This is a Wild Area event held by Sword and Shield. This event gives players a chance to capture the Alolan Starters.
An interesting note, is you could try to catch them with other types of Poké Balls, which couldn't be done previously.
Only the files normal_encount, normal_encount_rigel1, normal_encount_rigel2, and dai_encount were updated to Index 87.
To import these files into your save, use PKHeX's block importing function. Our tutorial should guide you through it.
Our first post of this page contains some data relevant to the importing stage.
There's info and some common questions regarding Wild Area Events here.
To see other Wild Area Events, here.