How do you decide what a monster will sound like?

It's the one question I am asked most when showing animations with the sounds I made for them. For a question that is asked so often, I always find it very difficult to come up with a suitable answer.

How do I decide indeed?

The problem in answering the question is that "decide" assumes a conscious process; that there are few options for me to pick from. Since most of the monsters and creatures from The Chronicles of Spellborn do not exist in our world, I cannot fall back on using samples of these.