Mephits are not native to the Gurth that is known to men and women. They inhabit other planes of existence that match their elemental inclinations.
They are 4-5' tall, under 100 lbs, intelligent enough to coordinate activities, and most are devious and dishonest.
Fire mephits were encountered in Darkness Rising in great hoards that live underground. They have a 4 yard, d+1 fire breath weapon, once per day.