There are currently 24 monsters that utilize the Poison Element
As of the 7/16/2012 patch, all poison monsters share the following qualities:
The Poison Monsters
When battling a poison based monster in the wild, your most effective fighter will be of the Ice element, as Poison is weak against Ice, and Ice is strong against Poison.
This battle combination will force your attacks to have a 1.5x multiplier attached to them, while incoming attack damage will either be cut in half, or the opposing monster will use a Normal attack if it has one.