Mew was hidden as the final entry in the original Pokédex.
Mew is National Pokédex number 151, following Mewtwo at number 150. In the original Pokémon Red, Green, and Blue games, Mew's data existed in the game, but it was not normally obtainable through the standard story or ordinary wild encounters.
Game Freak added Mew late in development, and its existence was initially kept secret. Nintendo and Game Freak later distributed it through special events, which turned Mew into one of the earliest examples of an event Pokémon.
Mew is often confused with Mewtwo because their names and Pokédex positions are connected. Mewtwo is a genetically engineered Pokémon created from Mew's genetic material, while Mew is the older, mythical species and the final entry of the original 151.