Dragonball Super is nearing the (apparent) climax of the Universe Survival Arc. In this arc fighters from all of the lowest scoring universes will duke it out in a Battle Royale to survive. The team with the last man standing saves their universe and the ultimate winner of the free for all gets a wish.
Super going down the multiverse path for it's celestial bureaucracy has led to a few new developments, however. There is a penultimate creator and destroyer above all the universal gods with the power to erase and create entire universes. There is a mortal that even a god cannot kill. And there are universes that share entire species as they are paired due to similarities. But one new development breaks that mold.
Universe 2, which is not the twin of Universe 7 (that contains Goku, Vegeta, etc) has at least one member of a species seen in Universe 7 already. The Yardrat. This species taught Goku the instant Transmission technique before his return to earth to train up for the Androids. They had their own planet and civilization. And now we see one in an unrelated universe.
There are 5 main theories that explain this:
1 - It's Toei. They broke their own rules/logic and just put him in an unrelated universe. Totally possible, but I think it's not the case. I'll explain why in theory 5. But they are the company responsible for GT so anything is possible. It takes talent to take Dragonball down a not fun path and they have that talent obviously.
2 - The explanation given to the characters about universal pairing is false. There are actually countless universes that started in roughly the same state to be curated by their gods and though other species are the prime ones in other universes they all share the same species. This can be further explained by Majin Buus impact on Universe 7. It is canon that Buu destroyed countless worlds in the past. And it's stated by the Supreme Kai in Super that the number of inhabited worlds is very small in Universe 7. This does contradict some prior arcs as Freeza was a planet broker and presumably he would've needed many customers to run this business as well as many targets. But then, his impact would've been similar though less pronounced than Buu on that extinction rate so he could be another contributor.
3 - The explanation is true but this lone Yardrat has managed to hop universes once. Striking a deal before the tournament someone managed to poach this fighter from Universe 7. Or at some point in the past he simply found a way to universe hop once.
4 - The explanation is true, but the Yardrat are a multiversal species. It is referenced in Super that Zen-Oh has destroyed universes in the past. Through some unknown means the Yardrat species was left homeless rather than destroyed and simply escaped to other universes at that time. It seems unlikely that they would've had warning given how quickly Zen-Oh is shown to work his destruction. But perhaps a lone exception allowed to live and relocated to other universes.
5 - This I think is the most likely. The Yardrat are a true multiversal species because of Instant Transmission. It has been shown throughout Z that not only can Goku teleport around the living world but also an alternate realm or plane within his universe, the world of the dead. When he teleports Cell to the planet of King Kai, when he visits Gohan on the planet of the Supreme Kai, etc. Goku can shift between planes within a universe. He only just learned of other universes but obviously that's not a universal state for things. In other universes we've seen awareness of the multiverse and an inability to travel to other universes. Hit asking for a cube ship as a reward. The riots that the Trio was observing. It is possible that other species or players in the universe know of the multiverse.
Instant Transmission hasn't been tested in this capacity by Goku that I'm aware of. But there's no reason to believe it wouldn't function to allow him to go to another Universe yet.
This is the most likely theory because it's the best business decision. We've already seen a desire to revamp old techniques with the Kaioken. This may just be the next in the chain. But also, it opens Super up to arcs that are in new universes, new worlds, and most importantly, without the gods tagging along. Up to this point Goku and company are only able to travel outside of their universe with the help of the gods and they are the strongest mortals in theirs. Super seems to be pretty obviously heading down the path of rivaling the gods themselves. But it needs buildup. To enable that they open up travel solo to entirely new opponents and use the existing relationships built in the tournaments to start it.
I think it's inevitable that the Saiyans of Universe 6 face a threat and Goku and company go to help as the next arc.
Thoughts? Alternate theories? Reasons one of the ones up there is more likely than 5?