>>42375533I do subscribe to the idea that there are legendaries that come in multiple and legendaries that come in single, but I think it's a little more complicated than that. We know Ho-oh is responsible for the manufacturing of the beasts, and we know that arceus is at least responsible for the creation and lake trio, so I think the situation is this:
Legendary/mythical pokemon are above a certain threshold of being, a state of being that does not equal non myth/leg mokemon of the same combat power. This level of being allows them to create others of, or uplift existing others to, a similar state of being, but certain pokemon are only made by certain others, either because they are the only one who knows how, or are the only ones willing to, make these specific other ones. Just as well, it is possible that legendaries cannot replicate themselves perfectly, which may explain why every instance we've seen of one creating others is always on a more-to-less line of power.
Any double instance of raikou was probably the fault of the same ho-oh, for instance, and every instance of the creation trio was probably the fault of the same arceus. Since it may or may not be true that arceus crated the universe it occupies, there's likely only as many arceus out there as there are universes, but we do know that arceus can produce multiples of the creation trio freely. There is also the situation of lunala/solgaleo, which are "legendaries" but operate under an evolution system. They may be self-creating, either one using this power to create new legendaries exclusively to create cosmogs, which are in herently less powerful beings than their parents, but through evolution turn into new solgaleos or lunalas of equal standing, essentially being a loophole in the "cannot self-replicate" idea I observe.
It's entirely possible that one-of legendaries (like maybe groudon or darkrai) could come in multiple too, but that would depend on whoever made them making more, I figure.