I am writing a fairy tale style story in which a (land based) creature is attacking seemingly random people within a village. The trick is, I want there to be a pattern to the people that the creature targets. The goal is to have the main character eventually figure out the pattern and use it to confront the creature. Some of the criteria I've come up with so far is:

* Creature only attacks either those who match the specific pattern or those who attack it
* Pattern is generic enough that there are almost always victims, but specific enough that most people are safe
* Pattern should not be immediately obvious, but it should also be difficult to spot without looking at all the victims together (creature has existed near village for a while without other people seeing the pattern)
* Pattern needs to be something that a normal human could discern without modern technology (so things like blood type would be out)
* Pattern is preferable based on something that doesn't fade upon death (so body heat would be out for example)

The village itself will be medium sized with woods nearby. It will not have technology, and though the world will have magic, the villagers will only be able to do small things with it if they can access it.

What pattern should the creature follow when hunting its victims?