It appears that the blog post where I found the pattern no longer exists. However, the pattern designer has uploaded the pattern to Ravelry, so if you are interested in making one (or getting someone to make you one) you can find the pattern here.
Because there are two, I can show both the back and front in one picture.
Yes, it is supposed to look like your head is being chomped by the hat. With teeth and everything...
From the back you can see the fin and the tail.
I mailed them off on Wednesday. They arrived on Friday. And I was sent a pic of the twins wearing them on Saturday.