my first Idea is most likely. an evil person *cough cough rose cough* has brought them all to Galar. so he could gain more power, for some reason. and they all escaped and they live here now.
my second Idea: clones.
now look at type: null. he is not on that list because he is already in this game. some scientists, according to the Pokedex, stole the information to make type: null. why would they need this? well to make more clones. they can't take Mewtwo's info becuase well, it was burned. so in order to make pokemon clones, they had to take the next best thing. Type: null's info. remember pokemon the first movie: Mewtwo strikes back. They made a mew clone. but mew doesn't look like Mewtwo, and Mewtwo doesn't look like mew. now the three new legendary birds ( if you haven't seen them I made an Image with them in it) They don't look like the three legendary birds. they even have different types. now if you're thinking what I'm thinking. I don't need to speak any longer, but 2% of you who might not be. let me break it down.
Mew=Mewtwo
(the following are made-up names I will use until further notice)
Moltres=moltwo
zapdos=Zapdous (see what I did there)
articuno=articunotwo (can't come up with a cool name for her/him)
"but Elite, What about the two new regi's"
I'm getting there.
now, what better way to practice cloning pokemon then by making your own. meet the two Regi's I like to call them Regishock and Regidrac. One of them is obviously an electric type and the other I'm thinking a dragon type (comment on what you think his type is if it's not a dragon.) so they took DNA from other pokemon to make these pokemon. If you look at the arm on Regidrac It kinda looks like a Charizard head. ok so. and Regishock, he doesn't look like anyone. except one pokemon that is used every day in the lives of Galarian people. ROTOM. so those are where they took the DNA from the pokemon. Dragons are not natural minerals, and electricity wasn't used in ancient times, so obviously they weren't made by the Regigigas him freaking self. so they have to be man-made.
and one pokemon that is so largely rumored to be In pokemon Sword and Shield, I have seen my self. Mewtwo. hmmm... now, why would he be there. obviously he has nowhere better to be, or he was kidnapped using these super awesome clones. and that would explain why some of these pokemon that are only in one part of the world and are the only one of there kind have shown up to galar.
Caption this Meme