Well there's three examples of Mystic-type monsters present in Zeal, if we count alternative colored sprites as being relatives to each other (Makes sense).
Stone Imps, Gargoyles, and Djinn (Ghul too). All three of those monster types are seen in Magus' army and the first two are seen living in Medina.
There is also one possible Mystic-type monster in 65mil BC as well. The Aecyto Weevil. This would only count if you group the Rhino Weevils in Heckran cave into the Mystics. Considering most of the monsters in the cave consist of bats, octopus monsters, and other creatures, I'd count this as a weak link.
So with that, it would seem that most of the Mystics came about in Antiquity. Imps, Gargoyles, and the sorceror-type Djinn monsters. But again, without any concrete explanation of their origins from legit sources (Kato's mouth or the games themselves) we can only speculate how they came to be.