It’s a first, or most anterior, rib of a Beaver (Castor canadensis). But there is a slight resemblance between the head, neck, and tubercle of the rib and the head of the seahorse. When identifying animal bones, the ribs start looking pretty much alike after awhile, but the first rib of a mammal usually jumps out at you. It tends to be shorter and straighter than the other ribs (photo below). Often ribs are not identifiable to species, but the unique shape of the first rib of this Beaver led to a quick identification. 

So the next time you run across one of these weird ribs you can tell your friends “Hey, it’s one of those seahorse ribs!”.