Unlike a character's instincts, monster instincts are not for making mundane tests or causing drama. They are simply there to better explain how a creature behaves. This post will (hopefully) provide some insight on how to effectively write them for that purpose. I'm going to begin by quoting the rulebook, a passage which has already (partially) appeared in this series of posts.
"Give every single creature in your adventure a want. Some will want something from the adventurers: their flesh, their souls, their help, etc. Some will want something from the adventure area: a lonely ghost wandering around trying to find his lost boots, kobolds who covet the dragon’s hoard, a giant spider waiting for orcs to blunder into its web."
Instincts are a fantastic way to express those wants. If you've built all of the other aspects of your creature already (and I recommend you do before thinking about instincts), you should see how the various parts add up to your concept. Figuring out what your creature wants may be as simple as putting those pieces together or focusing on one or two. It may also be more complicated than that. In those situations, I find it's best to just start asking questions.
- What does this creature want?
- If threatened, what does this creature do?
- Does this creature have a natural enemy? If so, how do they deal with them?
- What is this creature doing on an average day?
. . .and so on. Remember, these are instincts. If you ask the question above and you've still got nothing, the issue may be a lack of clarity of concept. Take a step back and think about your creature outside of the realm of rules for a game and then try again.
Let's end on some examples, using the passage above as inspiration.
Always skin your victims.
Always carry an empty soul gem.
If I can convince others to help me, I can finally succeed at this task.
Never leave a piece of gear behind, in this life or the next one.
Never break camp before counting the gold.
Always let your food come to you.