I haven't inserted any mythical beasts into my world...but I did add a whole freaking plethora of what I call Spirits. They're really more like gods, though. I also created a race of "Polifers"--humanoid animals. While I did keep regular animals in the world as well, the Polifers add a fun...
I don't think that they would have much of a culture. Maybe they take on the culture of whoever hires them? Also, I think leaving them (as individuals) nameless does move them closer to being a blank slate.