This dark snaggle tooth is small for the species.
The Hemipristis serra or more commonly known as the Snaggletooth shark roamed the ancient oceans with the megalodon shark, sperm whales, and other ancient creatures.
There is a modern day snaggletooth shark but it's ancient ancestor was much larger, and much more ferocious.
These hemipristis teeth have a distinct shape and due to the large serrations in relation to the overall tooth size look rather fierce and menacing.
The video below shows a snaggle tooth being found underwater, it is not the tooth for sale above but gives you an idea how these snaggle or hemipristis teeth are sometimes found in the dark water rivers.