1. Name
2. Biology (Appearance/ humanoid-or-not, etc, etc)
3. Size
4. Diet.
5. Behavior/ personality.
6. Malevolent/ benevolent/ chaotic/ other
7. History/ backstory
Personally, I think the best monsters and creatures are the ones that are the most human. What's more scary? A werewolf, where you can stand there and say "Yes, that's a werewolf", or a humanoid creature, where you're not too sure, but the slowness of its movements, its strange behaviour, the ever-so-slightly-off appearance makes you...wonder?
The best monster is one where you think that it could very well be real and waiting for you when you come home. And that you should always look behind you when you're on your own...