The Pokémon that became the first Mythical Pokémon distributed through Nintendo events was Mew.
Mew was added to the original Pokémon games late in development and was not normally obtainable through ordinary gameplay. In Japan, CoroCoro Comic publicized a 1996 “Legendary Pokémon Offer,” through which selected players could send in their cartridges and receive Mew. This made it the earliest major event-distributed Pokémon.
Mew’s rarity helped establish the event-Pokémon tradition: players could obtain special creatures through promotions, conventions, retailers, or later Mystery Gift services. The species is classified as Mythical rather than Legendary because its availability has traditionally depended on special distributions rather than standard in-game encounters.
Celebi, Jirachi, and Deoxys were also distributed through events, but they appeared later in the franchise. Another common mistake is treating Mew as a normal hidden encounter in Pokémon Red and Blue; it cannot be legitimately caught there without an event, transfer, or exploit.