I'm not too sure; sometime the characters look like some existing ones, but modified a bit, you should check the "starting pic" with the rules and everything (they often do this kind of stuff)
Most of the time it seems to be very freely interpreted, and you occasionally even see the artist put an オリジナル tag on, but on the other hand there is some basis for it, so it's not 100% original. I'm not sure how I feel about it...
Guys, I've been wondering. What exactly is Pixiv Fantasia anyway? All I know is that it originates from Pixiv.net, but is it some kind of "fan-created" universe?
Yeah, I think that's it. I don't know exactly how it gets started, but here's a page with an explanation of PFII, somebody who knows Japanese might be able to help out:
Well, google-translating that, it seems that it's indeed some sort of fan-made universe where there are different "Guilds" that an artist may join. I don't quite get if these Guilds basically are theme-based or something, considering that I see all sort of artworks (fantasy, mechs like this very one artworks, mix of both, and so on). Anyway, it looks interesting.