The Mystics

The Mystics appear in very different shapes and forms on the planet. Some are tiny gnomes others are giants.

Origin

They seem to be descendants of the reptiles or maybe even created by the scientists of Zeal (but that's a pretty weird theory).
Human-like Mystics like Flea or Slash are rather rare so it is possible that there are hybrids among them (the results of rare human/mystic relationships).

Language

While there are no hints in the US-version that Mystics may have their own language, there are some in the Japanese version. Some of the Mystics "speak" in Katakana instead of Kanji/Hiragana - it gives the impression they have either their own dialect or that they have some difficulties to speak the human language.

Magic

The Mystics seem to be more earthbound than the humans. Because of this the use of magic is more common to them. However, only the wiser can use magical powers.
While mankind is also capable of magic (surely because survivors of Zeal and "Earthbound" gave birth to children) most of the humans have forgotten how to use the arcane powers.

And the future?

In the distant future there are some strange mutants left, that haunt the ruins of the old human cities and facturies.
However, I'm not sure if they can be related to the Mystics of Crono's time. So maybe their civilization was destroyed shortly after the rise of Lavos or mankind itself destroyed them a long time ago. Since mankind is known for destroying everything strange or unknown, the latter is more plausible.
Those who are left have either joined Lavos' forces or try to survive on their own in a world that is already dead.