I think that pokemon who have a unique creation (Mewtwo, legendary beasts), or those that are simply super-powerful (Regigigas, Zekrom, Arceus) would be one-of-a-kind, while those that are less significant (legendary birds, swords of justice), or who are created in such a way that many exist (Legendary Golems, Phione) are just very rare. I ignored how many times they appear in the games, although what they do within the games is considered.
My lists:
My lists:
Mewtwo
Legendary Beasts
Ho-oh
Weather trio
Lake Guardians
Cresselia
Manaphy
Regigigas
Arceus
Victini
Tao trio
Legendary Beasts
Ho-oh
Weather trio
Lake Guardians
Cresselia
Manaphy
Regigigas
Arceus
Victini
Tao trio
Legendary Birds
Mew
Lugia
Legendary Golems
Eon duo
Jirachi
Creation trio
Darkrai
Phione
Heatran
Shaymin
Swords of Justice
Forces of Nature
Meloetta
Genesect
Diancie
Mew
Lugia
Legendary Golems
Eon duo
Jirachi
Creation trio
Darkrai
Phione
Heatran
Shaymin
Swords of Justice
Forces of Nature
Meloetta
Genesect
Diancie
Celebi is said to have the ability to travel through time. If true, this means that one Celebi could appear whenever it wants to in time, including possibly multiple places at the same time. Due to this, it would be impossible to tell how many Celebii exist, not unless you could view all of time yourself.
Deoxys is said to be a virus from space that mutated. It could be that the some of the other viruses on other worlds mutated into Deoxys, too, or that this Deoxys is the only one. Therefore, one could not know how many Deoxys exist.
Xerneas, Yveltal, and Zygarde lack any sort of information about whether they are the only one, or a few of a very rare species.
Deoxys is said to be a virus from space that mutated. It could be that the some of the other viruses on other worlds mutated into Deoxys, too, or that this Deoxys is the only one. Therefore, one could not know how many Deoxys exist.
Xerneas, Yveltal, and Zygarde lack any sort of information about whether they are the only one, or a few of a very rare species.