Over on Ravens & Pennies, Christopher Rice has an excellent post for people making monsters for their GURPS games.
Gamemaster's Guidepost: The Art of Creating Critters
It's a great guide. If I could usefully add to it:
How skilled is it? Know what its absolute skill level can do. But also know what its skill level gives it relative to its intended foes. Skill 16 is murder vs. Skill 12, but it's toast vs. Skill 20. All the instant death ST-based damage in the world is harmless against foes you can't hit.
Does it have special attacks? Especially for genres like DF, you need to know what special attacks it has that make it especially able to deal with its expected opposition. If its attack forms are limited in a damage type (it only uses fire, say) know how common totally effective counters will be.
Know the synergies. In other words, if your critter has 20 HP and Regeneration, know this doubles the HP regen rate. Know how Unkillable 3 and Recovery will work together. Familiarize yourself with what all of the abilities you are putting down on the sheet meet and how they interact with each other.
I think those, coupled with Christopher's advice, will help when you are making up GURPS monsters.