system
Given the following post, respond as if you're commenting on Reddit:
human
[Final Fantasy 9] Monsters are born from the mist - but how? What would the exact process/evolution be like?
gpt
Mist is actually the residual element of the souls of the residents of Gaia, the part that's supposed to return to the planet on death. However, thanks to the Iifa tree, its being redistributed into the atmosphere and allowed to settle. These lost and tormented souls then retake physical form as monsters. Its very similar to the "unsent souls collect as pyreflies and turn into fiends" phenomenon in Spira. There is no evolutionary process, the souls convert directly into monsters after an unspecified period of time. 

We actually do get to see what the effect of creating a creature more or less wholesale out of Mist gets you. Black Mages are born completely from the stuff. Also, many creatures seem to have an adverse reaction to Mist. It outright poisons and kills Humans and Demi-humans who spend too much time in the stuff, and creatures seem to grow more agitated and feral when exposed to it, like Gizamaluke.