Pokémon: The First Movie (movie 1)

The newly added CGI footage and effects (Example: The ominous cloud barrior surrounding Mewtwo's lair) were created originally for the japanese DVD release of Mewtwo Strikes Back.

Currently, the highest-grossing cel-animated film in North America.

Winner of "Best Animation Film Award" at Animation Kobe '98

The names of the three Pokemon trainers (two boys, and one girl) who came to Mewtwo's island before Satoshi, Takeshi, and Kasumi were never mentioned in the film. They are identified in the ending credits or in some promotional material. 

The bearded scientist in the beginning of the film is Dr. Fuji. 

The American version didn't include the prologe called "Origin of Mewtwo" for it's theatrical run due to its dark nature.

A narrator and Mewtwo's voice were added in the American version, in the first segment of the movie, to help better explain the movie to people who may not have had such a wide knowledge of the Pokemon franchise.

The original Japanese version of the movie had a different musical soundtrack with almost no vocal songs, and many scenes that do not feature any music at all. When this film was dubbed into English, the music was also re-scored and was also added to the scenes that originally had silence in them.

This marks as the first anime appearance of the Pokemon, Donphan(Whom Bulbasaur fought against in the intro). It's name was never mentioned in the entire movie. The games the Pokemon originated from were Pokemon Gold & Silver Version. Both of the games weren't released in America at the time this movie was released. So most American fans were puzzled by seeing the sight of the then unknown Pokemon in the film.