The "Evil Demon" thought experiment was invented by which philosopher?

The answer is Descartes.

Descartes, a 17th century French philosopher, proposed the idea of the "evil demon" or "malicious demon" to explore the idea that if there exists a being advanced enough, and beyond the realm of human understanding, he could create a completely false world for us to occupy. Therefore, humanity should reasonably doubt the reality everything we experience.

This is especially true if we subscribe to the idea of beings as powerful as the one described, which virtually everyone at the time did (God).