Although Shiny Pokémon were first introduced in Pokémon Gold and Silver (remember that majestic Red Gyarados?), Pokémon Sword and Shield is the first appearance of what some fans may call "Square Shinies."
Recently, a dataminer found that out of all Shinies that can spawn, 15/16 of them are regular Shinies, and 1/16 of them are these new Square Shinies. So don't worry, the regular/Star Shinies are still in the game and they seem to be the most common type of Shinies to encounter.
So what's the difference between these two types of Shinies? Well, first we have Star Shinies or the Shiny Pokémon that differ in color and feature stars when Pokémon pop out of their ball in battle, when they are hatched, or when they are found out in the wild. Regardless of the type of Shiny, the odds of obtaining a Shiny Pokémon are 1 in 4,096. Once you've obtained a Shiny, the odds of it being a Star Shiny are 15 in 16.
Then there are the Square Shinies. These Pokémon have square shapes that surround the Pokémon instead of the common stars, otherwise they can be obtained the same way Shiny Pokémon can. They also feature a lens flare within the special animation when sending your Square Shiny Pokémon into battle - very cinematic. Once you obtain a Shiny Pokémon, the odds of it being a Square Shiny are 1 in 16, which means the chance of encountering this Pokémon out in the wild is 1 in 65,536.
There are other ways of increasing the likelihood of obtaining Shinies, such as the Masuda Method, Shiny Charm, chaining Pokémon, and mass breeding Pokémon. Chaining Pokémon involves encountering the same Pokémon over and over again in order to increase the likelihood of it being Shiny. Some say that the odds of a Shiny being Square or Star are flipped when Pokémon are chained. In other words, if you battle one Pokémon over and over, the odds of the type of Shiny Pokémon you encounter becomes 15 in 16 for Square Shinies and 1 in 16 Star Shinies. Others say that if you choose one method (such as breeding) over another, the Shiny could be destined to be a Square Shiny only.
There has been some confusion about the data and what it means, so only time will tell what the true math is behind encountering these Square Shiny Pokémon.
Pokémon Sword and Shield are out now for the Nintendo Switch, will you be the first to fill your 'Dex with 400 Square Shiny Pokémon?