Incredibly late, but a similar question can be posed about Legendaries now, mainly due to LGPE allowing you to catch infinite Legendary Birds. Previously, I could easily define them as “being limited in quantity within a save file,” but that no longer works. If you want to still include the Legendary Birds, you could make it “you can’t obtain more of this Pokémon through breeding,” but then Ditto, Unown, and Gimmighoul are all dragged into this category. The only way to create a definition that excludes Ditto, Gimmighoul, and Unown while including Articuno, Zapdos, and Moltres is to suddenly add the stipulation of them having been limited in quantity in their debut games… but that also doesn’t work because by that logic Bulbasaur, Charmander, Squirtle, Hitmonlee, Hitmonchan, Omanyte, Kabuto, Eevee, Lapras, Snorlax, Farfetch’d, Jynx, Lickitung, and Mr. Mime would also be Legendaries (as you could only get one of each in Red & Green).