The creature known as Mew in the first Pokémon movie, Mewtwo Strikes Back.
A video recently posted by the Youtube channel Did You Know Gaming produced some freshly translated blog posts, originally written by the Pokémon anime's first chief writer, Takeshi Shudo. In these blog posts, Shudo talks about the production of the first movie and how Mew was originally written to speak, just like Mewtwo (and many other Legendary Pokémon later in the anime). What may surprise fans most is that Mew's dialog was not nearly so cute and sweet as its looks.
Mew Actively Insulted Mewtwo in Pokémon's First Movie
According to Mewtwo's responses to Mew weren't substantially altered; thus, it's fair to assume that Mew is still saying some of those hurtful things to Mewtwo, just in a language that humans can't understand.
Mew being rather mean is an interesting turn, as mew's cruelty isn't obvious in the final version of the film. However, Mew's original characterization hasn't been forgotten; as Goh manages to impress it enough with a display of thankfulness to get it to stop. Still, Mew very nearly wiped out the entire expedition party, all while having what it saw as fun with them, which seems perfectly in line with the cruelty it showed Mewtwo in early versions of that film.
Mew Was a Fairy Pokémon Before Fairy-Types
In that sense, Mew is quite a lot like real-world legends of fairies; a strange and powerful being whose looks can be deceiving, that one is better off not getting involved with. Fairy-like attributes have become a hallmark of Mythical Pokémon that followed in Mew's footsteps, like Celebi or Jirachi, so this seems to have been a deliberate choice, even if Mew's cruelty has been toned down over the years.
Fans interested in learning more about Mew's original characterization in Pokémon's First Movie should watch Did You Know Gaming's video and see for themselves.
Source: Did You Know Gaming
