I have also seen this . One is a whitetail buck that came through my taxidermy shop several years ago. I have the cleaned muzzle of that deer stored away in a box somewhere. Another one is a mule deer buck skull that I found while hunting, it was missing one of the canines but the hole is there and the other one is still intact. I still have that skull. I once read an article about this and it is a genetic throw back to prehistoric times when deer actually had fangs. Look at an elk skull and you will see that the ivories in elk are in the same location as they are in the deer skull. The ivories in elk are the remnants of fangs that prehistoric cervids had. I have never seen an elk without these ivories and it is very rare for deer to have them but they sometimes do. Today there are deer species or distantly related to deer species around the world that still have fangs. Do a google search for "deer with fangs" and you can see picture of them and read all about them.