Sort of redundant, but for roleplaying purposes, this villian is a Serpentfolk Sorcerer with the Serpentine Bloodline.
The first level power of the bloodline is "Serpent's Fang"
At 1st level, you can grow fangs as a free action. These fangs are treated as a natural weapon inflicting 1d4 points of damage plus your Strength modifier (1d3 if you are Small) plus poison (Bite—injury; save Fort DC 10 + 1/2 your sorcerer level + your Constitution modifier; frequency 1/round for 6 rounds; effect 1 Con damage; cure 1 save). At 5th level, these fangs are considered magical for the purpose of overcoming DR and the poison damage increases to 1d2 Con. At 7th level, your poison requires 2 successful saves to cure. At 11th level, your poison damage increases to 1d4 Con. You can use your fangs for a number of rounds per day equal to 3 + your Charisma modifier.
The Serpentfolk race, already has fangs with their own (different) poison.
bite +5 (1d6–1 plus poison)
Poison = Bite—injury; save Fort DC 15; frequency 1/round for 6 rounds; effect 1d2 Str; cure 2 saves.
The save DC is Constitution-based.
So when they use their Serpent's Fang ability does it completely suppress their natural fangs for the duration? Use the higher of the two damages and both poisons? I assume it is a polymorph effect basically and I'm thinking it would just replace their normal fangs in all ways.