Flea's the only one I'm not sure about.
Who is?
Actually, official artwork of Flea shows, ahem, him to have pointed ears similar to Slash and Ozzie's, or maybe even Magus'. One could go on with theories about Flea being a magically (and perhaps not just in that area) enhanced human but I won't get into that as I have no evidence to support that, and the one theory I just though up isn't a pretty one.
Mystics were named "Mazoku" though (I think it means monsters or something like that) in the Japanese CT. That's also a name used in Dragon ball.
Don't know about the Japanese name, but I do remember Joe the piano man. "Monster" is a generic term in RPGs, as we all know, though one could read into the statement as a term describing a demographic like non-human sentients. "Demon" or "youkai" in the anime Inuyasha actually translates closer to nature spirits than the more negative connotations used in the English dub, as I've heard fans point out.
Monster makes demi-human sound better by far, but maybe Joe is an exception and other Kilwalas are anti-social and violent towards humans. We don't know. I'm not sure what year Joe was found in ( 1,000 A.D. I think, correct me if I'm wrong) or the appearances of the wild Kilwala in the game besides 65, 000, 000 B.C. There the little fuzzballs could be found dancing around a rock in a sort of ritual, so the species was apparently intelligent enough to recognize repetition of an action.
So Kilwalas, or at least the Family of Kilwalas, has been around since the times of the Reptites and early humanity. Whether they were sentient then or evolved to be we don't know, but the fact remains an intelligent species that has been around since the dinosaurs is (probably) lumped together in standard conversation with the Mystics that followed Magus, at least racially. There may not share genetic ancestry at all, but they do have one thing in common; they aren't human, the dominant race on the planet. No Kilwalas fought you in Medina in 1,000 A.D., and did anyone see the Kilwala Brigade stationed in Magus' Castle?
They were pretty tough. (Just kidding.)
I don't recall the term monster being used to describe demi-humans in CC, but I vaguely remember that the villagers in Arni may have called Serge (Lynx), Harle, and Sprigg that after his change (and even with Radius in the party! The chief may have whacked his buddy, but he was not intolerant.)
In short (too late for that, I guess) I submit that "monster" is probably a generic though not politically correct term for non-human, sentient creatures. "Demi-human" is similar, though more PC in its use. As for Mystics, I'm thinking of another example in gaming: the Covenant in the Halo games, a religious order of various alien races that also has a hatred of humanity.
Halo 2 rocks, BTW.