Every Mythical Pokémon Explained

Mythical Pokémon are the rarest Pokémon in the entire series. From a secret Mew hidden in Red and Blue, to a GS Ball locked away for 17 years and an Azure Flute that was never released, here's every Mythical Pokémon, what makes them special, and how people actually got them. Which Mythical did you never manage to get? Let me know in the comments!