There's been an more in-depth discussion of the success and failure rate for players looking for rare drops, It can be found here:
viewtopic.php?f=4&t=10279&hilit=binomial
For this example, let's presume the drop rate of the fang is 3%, and each attempt is each Mamba killed (and keep in mind the limited number of Mamba's in each area). Note at 3% drop rate, 54% of players will still not have gotten the fang to drop after killing 20 Mambas, and 5% of players will still not have gotten fang after killing 100 Mambas. In contrast, 3% percent of players will have gotten it on the first Mamba killed.
So a small handful get the fang the first try, a small handful are left with nothing after 100 Mambas... rather brutal spread.