Around the world, there are several roughly polygonal coins. Here's an example:
One thing you'll notice is that they all have an odd number of sides. It turns out that this is universally true for modern polygonal coins (if you have an extant example where this is not true, please post it!!)
And it turns out that there's a perfectly sensible reason for this.
Why do modern polygonal coins have an odd number of sides?
Edit: I've been convinced by the various answers and comments that my statement is not correct and many coins around the world are, in fact, genuine polygons. My inspiration for this puzzle (if it's really a puzzle) comes from here.