In the novel I'm currently working on, most of the characters aren't human. One of my goals for this book is to remember that as I write. It seems like it ought to be an easy thing. Nonhuman characters show up all the time in speculative fiction: vampires, were-whatevers, the Fey, aliens. But quite often, what shows up on the page isn't a nonhuman creature that is truly Other, but a sparkly sexy human, wearing a vampire costume.
The point of putting a vampire in a story (unless the point is, vampires are dead sexy, in which case, sure, but so are a lot of people) is that the character functions in the story in a way that a human character does not and cannot. You don't write a character as the Faerie Queen because you need a person who is really pretty and who a lot of people are obsessed with - you can bring a supermodel on stage for that. The point of writing a non-human character is that they are Not Human, and hence governed by a different set of needs, of behaviors, of rules. If a vampire doesn't have a soul, he should act like an amoral sort of guy. If the Faerie Queen doesn't have a heart, she may have lovers, but she won't have loves.
Normally, when writing, the challenge is to put actual people on the page, not just a set of quirks and characteristics. But the narrative interest inherent in nonhuman characters is their inherent nonhumanness. The challenge when writing something nonhuman is to make such characters recognizable, not as people, but as what they actually are.