Shape-Shifting fox ? We had one in the Old Forum, German-language section. I think his name was Renard, and he was in the first role playing topic here which could be found in the books in the "Larian Dungeon".
I'd say, take a look at antique, medieval and ancient creatures from scripts.
What I remember from an roman arc in a local musuem here is for example a creature with the first half being kind of a horse and the second harlf looking like a big fish. I don't know its name.
This would be a sea creature, I guess, and I wouldn't declare it evil, especially since I believe that not all monsters should necessarily be evil.
Some should really be friendly and talkative.
Celtic monsters often look interesting, too.
Just found this book with celtic patterns with a quick search.What I personally would find interesting is, to inclkude a monster that can later be actually found in an museum. Gamers (who hardly ever go into a museum, according to the cliché <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" /> ) could recognise it there, thus this would become kind of an easter egg. <img src="/ubbthreads/images/graemlins/winkwink.gif" alt="" />
Ah, by the way, have mermaids been introduced in fantasy games ? <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
http://en.wikipedia.org/wiki/Melusine