A massive portion of the Glazed Pokédex consists of Legendary and Mythical Pokémon spanning Generations 1 through 6. They do not appear randomly; their encounters work through dedicated post-game scripts. Location / Unlock Condition Complete the Fusion Labs sidequest; found in Wispy Ruins. Mewtwo
Static encounters (gift Pokémon, legendaries, in-game trades) were verified by scripting triggers. Example: obtaining Celebi requires completing the Ilex Forest sidequest after Johto Elite Four.
The Ultimate Pokémon Glazed Pokedex List Guide: How It Works & How to Complete It