Monsters/Creation
Core Principle
The most important thing to remember is that a monster is an alternative to a character, and should be about as useful as a character of the same point total.
Statistics
For specialties, consider how many a normal character with that cost would have.
Monsters may occasionally have higher or lower base statistics before Specialties are applied. Their unarmed attacks shouldn't be more damaging than good weapons, and more than +1 higher HP or Toughness is a little bit excessive.
Unique Features
Monsters often have disadvantages by comparison with humans: they may be stupid, unable to talk, lack opposable thumbs, be social outcasts, or merely lack the many many class bonuses a person gets. To compensate, it's good to give the monster something different it can do, so it can be interesting in non-combat situations.
Advancement
Advancement options for monsters are not necessary, but in essence, every allowed combination of advancements should also meet the standards of a well-designed monster.
