Well, this is still a widely believed tale about snakes with triangular heads are dangerous-venomous. This is not fully true. Yes, some venomous snakes do have what appears to be a triangular shaped head but we also have non-venomous snakes that have that same characteristic. Many water snakes have the ability to flatten their head to mimic the characteristics of a water moccasin. Rat snakes can also have what appears to be a triangular shaped head.  The venomous snakes with the triangular heads normally get that from the venom sacks located on the sides.