Consider the following 10 suggestions as exemplifying 10 "roles" usually found in RPG's.
1. Human -- "Ordinary" bandits, knights, wizards, etc.
2. Goblin -- A small, sneaky, malevolent, possibly fey humanoid.
3. Giant -- A big, smashy, brute.
4. Wolf -- A common animal threat. It fights in packs.
5. Dragon -- Self explanatory. If I were starting from scratch, I'd 86 the color wheel and divide them into 2 types -- Fire Breathing and Poisonous.
6. Demon -- An evil brute with supernatural powers.
7. Ghoul -- A corporeal, flesh eating undead being.
8. Ghost -- An incorporeal, soul stealing undead being.
9. Grell -- An unnatural, tentacled horror from beyond. More Howard than Lovecraft.
10. Golem -- A mindless automaton that neither breathes nor sleeps.
Special Bonus 11 - 15!
11. Bear -- A big scary animal that fights alone.
12. Panther -- A sneaky animal that hunts by stalking.
13. Giant Spider -- An enormous poisonous insect (yes, yes, I know).
14. Lizardman -- An inhuman, but natural intelligent creature. Survivor from a fallen empire.
15. Succubus -- An evil being that uses magic and temptation instead of brute force.