There are a lot of vampires, ghosts, and dragons out there in children and YA lit, but some creatures and mythological beings never make it to the title page:
1. Sphinx- we have changelings, dragons, and even fairies, but no one likes a riddle-loving lion who will gobble you up. Poor sphinx!
2. Mermaid- Blame Disney for this. No one wants to tackle these fish-tails for the same reason that they won't retell the Hans Christen Andersen fairy tale.
3. Muse- Neil Gaiman and all the other writers who have written about muses made these nymphs a fictional cliche. Best version? "Ballad of the Flexible Bullet" by Stephen King.
4. Zombies- not brainless zombies, mind you, but emotional undead beings who are trapped in suspended animation
And am I going to write about all of these? Heck yeah!