Ever since Nintendo's official announcement of Pokemon Sword and Shield, fans have been digging through leaked evolutions (mostly fakes), a new Bug-type Pokemon, region rumors, and more to get as much detail ahead of time as possible. The latest rumor to hit the internet, however, is that the new gen 8 games will contain a fan-favorite feature in the form of partner Pokemon.
For clarification, partner Pokemon are those who travel beside trainers outside the traditional Pokeballs. This was first introduced back in Pokemon Yellow, when Pikachu would follow the trainer around, and has appeared in a few games since including Pokemon Heart Gold and Soul Silver as well as the recent Pokemon Let's Go Pikachu and Let's Go Eevee.
As with all rumors, it's best to take this news with a grain of salt, but this rumor has more official backing than most rumors/leaks currently out there. But the origins of this rumor come from a since-deleted Pokemon Sword and Shield tweet straight from Nintendo of America that directly used the phrase "partner Pokemon."
How this manifests exactly remains to be seen, as it could be based on the first Pokemon in the party like Heart Gold/Soul Silver, but it seems more likely it'll focus on the starters in a manner similar to Let's Go. Either way, the tweet and the fact that it was deleted makes this rumor more plausible than the leaked Sword and Shield Starter evolutions that look like Sun and Moon "Ballio" fan art.
Another reason for it being deleted could be tied into the "Gym Master" phrase instead of "Gym Leader" or both phrases. These two unusual mentions could arguably be small semantical differences, but then that begs the question of why delete the post. If nothing else, this tweet likely means that there is also a new take on gyms in Pokemon Sword and Pokemon Shield, though if it relates to Sun and Moon's island challenges in any way remains to be seen.
Pokemon Sword and Shield release in late 2019 exclusively for the Nintendo Switch.