Mist Mephit

(from Planescape: Monstrous Compendium Appendix I - 1994):

Mist mephits fancy themselves spies of the highest caliber. They quickly report other mephits who show mercy or other treasonous behavior, and they seldom engage in idle banter with other mephits. 

Mist mephits can see clearly in fog or mist. Their skin is pale green. 

Combat: Mist mephits seldom engage in melee unless trapped. Their soft claws inflict 1 hp damage. They can breathe a sickly-green ball of mist every other round, up to three times an hour, that automatically envelops one victim within 10‘ (save vs. poison or take ld4+1 points of choking damage and blinded for 1-4 rounds). 

Mist mephits can create a wall of fog once per day, as a 3rd-level wizard. They can also assume gaseous form once per day, and often do so to spy on others or escape combat. 

Once per hour, a mist mephit can attempt to gate in 1-2 other mephits, either ice or mist. If two arrive, they are the same type. 

Powerful winds, including a gust of wind spell, cause mist mephits to flee in confusion.

(from 4th Edition Dragon Magazine #421 - 2013):

Mist mephits are an offshoot of steam mephits that prefer cooler climates in the material world, such as swamps and marshes that generate thick fog.

They have gray-white bodies that constantly secrete mist from their skin, covering the mephits in a thin layer from head to toe.

Their bodies sometimes look translucent, and they have the ability to become insubstantial and blend in to the haze that surrounds them.

Mist mephits are quite shy, preferring to stay at a distance and remain hidden in the darkness.