Are legendary Pokemon Unique/One-of-a-kind, or are they just all super rare species?

I posted a thread like this a while ago and damn, I've never had a thread last so long. But now I've come to a conclusion based on what I've been told. Feel free to critique/edit the list below as you please - I want input.
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NOTE:
THIS IS A DISCUSSION ABOUT THE GAME CANON ONLY!
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Birds: Definitely not unique, as they can be found in multiple regions. Battle Chatelaine use them as well.

Mew: Probably just very rare, as all Pokemon had to descend from it, meaning that there must've been a reproducing population.

Mewtwo: Unique (probably).

Dogs/Beasts: Definitely not unique. Entei's entry even says it can reproduce. Battle Chatelaines use them too.

Lugia: Since this is the game canon, the baby in the anime doesn't count. So unique.

Ho-Oh: Unique.

Weather Trio: Unique. Kyogre/Groudon literally created land and sea. Rayquaza is there to calm them. Kyogre/Groudon also have world-destroying powers (noted in OR/AS) so more than one couldn't exist.

Regis: Not one of a kind as they can be found in multiple regions. Battle Chatelaines use them too.

Regigigas: Probably not unique as it's offspring aren't. Also, found in multiple regions.

Heatran: Can be male/female so definetley not unique.

Cresselia: Found in multiple regions so probably not.

Darkrai: If Cresselia isn't, Darkrai isn't.

Arceus + Creation Trio: Arceus can create newborn versions of the Creation Trio, but there is only one in the wild. (Probably re-created after a trainer captures them.) TL;DR - unique with an exception.

Lake Trio: The lore makes them seem unique.

Three Musketeers: Not unique. Battle Chatelaines use them.

Keldeo: Since it's a "member" of the Three Musketeers, probably not.

Tao Trio: Lore implies unique.

Meloetta: Probably not unique. No reason for it to be...

Victini: The dex implies that it isn't unique.
(cont.)