Movie 1 Mewtwo was always a male character, it wasn't a branding thing like Lugia. He was intentionally modelled after the Phantom of the Opera, specifically his Japanese portrayal by Ichimura Masachika.
Mewtwo's lines were written in Masachika's voice, and production was able to secure him to voice the character.
>http://style.fm/as/05_column/shudo168>http://style.fm/as/05_column/shudo169>https://www.oricon.co.jp/news/2139441/full/