A long time ago I made a zombie hat with teeth for one of my friends. I can't find the pictures of it anywhere even though I'm pretty sure I saw it when I was cutting these out. Hm. Anyways, I thought it was a pretty cool idea I haven't seen used much, so I wanted to try it again (and simplify the construction some). Here's the result, two styles of zombie hat:
This first one is much like the original design. It's made of 4 panels and a bottom band, lined with another layer of the same fleece, and has the teeth sewn between the layers. On the original hat, I made the teeth by sewing them right sides together then flipping, but this leads to extra bulk in the points. This time I did a tight zig-zag around the edges. It seems to work fairly well.
The second hat is pretty much the simplest hat pattern ever, but I like how it works with the monster details.
Instead of just lining it, I made it reversible. The other side reminds me of the biting cat from Azumanga Daioh:
...or just some generic monster. Again the teeth are just sewn between the inside/outside layers.