>>42323441We don't know that though. For all we know, the original dragon is from space or it's an ultra beast and there's a whole planet full of them. I'd still agree that Reshiram, Zekrom, and Kyurem are unique by that metric, given that they were split apart in a strange manner that's unlikely to have happened to any other original dragons, should they exist.
However, just because there hasn't been lore about where the wolves or the original dragon comes from isn't evidence towards there not being more. Especially since Zacian and Zamazenta are both capable of creating illusions. There could be an entire island full of the things that's kept invisible by their collective powers, and the two we see in Sword & Shield are just a pair that happened to leave and did something of enough historical importance to be semi-remembered.
I'd place them as similar to Ho-oh and Lugia in that regard. Specific individuals of that species have been significant in Johto's history, and we don't know where Ho-oh and Lugia really come from given that they just used the towers in Ecruteak as a perch that they visited.
