Which magazine first revealed the hidden Pokémon Mew to the public?

CoroCoro Comic first revealed Mew to the public.

Mew was hidden in the original Pokémon games and was not part of the normal in-game sequence for collecting the first 150 species. Its existence became a major source of rumors among players, helping sustain interest in the games after release.

The Japanese magazine CoroCoro Comic publicized Mew and later ran a contest connected with distributing it. This gave players a legitimate way to obtain the otherwise inaccessible Pokémon, although only a limited number of people could receive it through the original event.

Mew is classified as National Pokédex number 151, immediately after Mewtwo. It is also described as the ancestor of all Pokémon because its genetic material contains the DNA of every Pokémon. That story detail is distinct from the real-world fact that CoroCoro helped reveal the character.