The latest Pokémon that will be appearing in Pokémon Sword & Shield has been teased under mysterious circumstances. The official website for Pokémon Sword & Shield has a glitched-out page for a Pokémon that has had a lot of its information redacted.
The mystery Pokémon is almost certainly an evolution of Farfetch'd, which fans have been referring to as Sirfetch'd. The reason why fans suspect that the new Pokémon is a Farfetch'd evolution is due to a leak on 4chan that gave out lots of accurate information about Pokémon Sword & Shield before it was officially revealed, including information about Dynamaxing and Galarian forms.
There could be more to this latest Pokémon reveal than meets the eye. This is the first time that Game Freak has revealed a Pokémon in this manner and it's possible that the glitch-filled tease is a reference to one of the oldest mysteries of the Pokémon series - MissingNo. In Pokémon Red & Blue, it was possible to encounter a glitch Pokémon that was created from random data within the game. The most famous name for this type of Pokémon is MissingNo. The reason why MissingNo exists is well documented by fans and the recently leaked assets of a prototype of Pokémon Red & Green have uncovered even more information about the why MissingNo appears, as it seems to be tied to Pokémon that were cut during the development of the game.
The fact that the latest Pokémon reveal uses glitched imagery is the first clue that it's related to MissingNo, as well as the fact that the sprite is likely meant to be flipped on its side, at which point it resembles the overall Tetris block shape of the most famous MissingNo sprite.
Pokémon Red & Green were originally meant to include a Bird-type for Pokémon, but this was scrapped during development in favors of the Flying-type. There is still information for the Bird-type Pokémon in Pokémon Red & Blue and the only way to find a Bird-type Pokémon is through creating a MissingNo.
The leaked prototype assets for Pokémon Red & Green still have a few unidentified slots for Pokémon, which have been analyzed on the Helix Chamber website. It has been speculated by a member of the Helix Chamber team that one of the remaining blank slots belongs to a scrapped evolution of Farfetch'd.
An evolution of Farfetch'd was definitely planned at one point, as there was one found in the leaked Space World '97 demo for Pokémon Gold & Silver, which was called Madame. The two leaked Pokémon games share scrapped Pokémon, such as the baby form of Vulpix, so it's possible that an earlier version of Madame was one of the scrapped Pokémon in Pokémon Red & Green.
It's possible that the scrapped evolution for Farfetch'd is the identity of one of the missing MissingNo Pokémon in Pokémon Red & Blue and that it was planned to be one of the original Bird-type Pokémon. It would be weird for Game Freak to start acknowledging the glitch Pokémon after so long, but maybe they will use it as a meta-joke excuse for why Farfetch'd can suddenly evolve after all of these years.
Source: Helix Chamber, Pokémon Sword & Shield