>>37158451There's a huge number of reasons for the Mewtwo-Giegue connection, but the most significant one is that many of the employees who worked on Mother moved over to Creatures, Inc., which was the successor to the previous company.
There are a number of other conceptual connections as well, such as the direct parallels between Giegue and Mewtwo's stories in both the games and anime. It wouldn't be surprising to me if Mewtwo was directly built on Giegue's model.
The influence of Mother even seeps into RBY, with one of the beta names for Red being Ninten. The only connection to Frieza is the color scheme at best, and perhaps the fact that he's somewhat villainous, but those are only very superficial connections.
I'd recommend looking into it more, it's actually very interesting.
