I may be going out on a limb here, but if you’re writing urban fantasy (or any kind of fantasy), the supernatural & paranormal characters of your story are going to need motivations, feelings, and possibly even emotions, just like your human characters. Whether these non-human characters are good or evil, they will require the same careful attention to detail as your human characters. And I’m not just talking about vampires, although Anne Rice’s Lestat and Stephanie Meyer’s Eduard Cullen are excellent examples of what I am talking about with regard to fully-fledged characters.

Even creatures that serve their masters are given added dimension when they are unpredictable. Dragons particularly seem very independent-minded; think of Christopher Paolini’s Saphira, who often countermanded Eragon; or even tiny Pantalaimon, the daemon familiar of Lyra in Philip Pullman’s Dark Materials trilogy. These were creatures that we came to love, not because we could see into their minds, but because they expressed themselves as true and complex characters, not mindless animatronics. It is interesting to note that Saphira and Pantalaimon show creatures with real emotional range, where the truly scary monsters seemed to be driven by emotionless instinct.
So keep in mind that readers want to read about fantastic, unpredictable creatures that are driven by motivations that are understandable. Your cryptozoids may be sentient or not, good or evil, but to be memorable, they need to be as well thought-out as your main characters. Where do they come from? What is their life cycle? What is their natural habitat? What are they afraid of? Why are they feared? Ground them in reality, and they will not only be believable, they’ll be memorable.