In regards to d20 supplements, Jade Dragons and Hungry Ghosts, a monster book by Green Ronin, contains quite a few chinese creatures. Unfortunately, it is a 3.0E book. There's also quite a few monsters in Dragon Fist that you can adapt, but the system is an AD&D variant.
For more literary sources, I would recommend reading Creation of the Gods, Journey to the West and A Chinese Bestiary. All three books portray a number of formidable foes that can be drawn upon for inspiration.
From those texts, you will find that the majority of creatures in the Chinese mythos are either dragons (virtually forces of nature, the most commonly depicted one, Ao(?), is a king who lives at the bottom of the sea with a court of aquatic creatures serving him; also, thier daughters often fall for mortals), shapechangers (often old enlightened animals such as Sun Wukong but also men like Erlang), undead (ghosts, vampires, and zombies being the most common), demons and gods (pretty much all of the outsiders could fit in this category, just adjust your descriptions; you should note that both Heaven and Hell work together AND both of thier ranks are filled with good-for-nothings), and strange species of men and animals (gigantic tortoises, men with no face, men with holes in thier chests, fishes with human heads, etc; so virtually any humanoid or animal species can be dropped in with effort). Exceptional individuals also have a tendency to possess various mutations (e.g. Nezha's numerous heads, Grand Tutor Wen's third eye, Thunderbolt's entire body, etc) and often have underwent some sort of religious training (typically either Buddhism or Daoism) to earn thier powers (Jiang Ziya, Erlang, Nezha, Sun Wukong, etc).
