Poison is one of the 7 elemental damage types.

Poison is the racial damage type of the Matis. The poison damage elemental spell can only be learned from a Fyros magic trainer once a mage reaches level 125. It is generally the preferred damage type to use while in the forest.

Matis player characters have a natural 20% protection against Poison damage.

To increase your chances of resisting a Poison elemental spell, you have to wear jewelry that raises your Forest resistance.

Cratcha are capable of casting Poison elemental spells.

Kirosta have a poison sting which does Poison damage, though the attack itself is physical and not magical in nature. That means Poison protection will protect you against it, but Forest resistance will not help you resist it.