The fire gecko is the most deadly kind of gecko in Fallout 2. They appear mostly in late game and, as indicated by their name, can breathe fire thanks to unknown toxic substances being combined in their gut. Those things are quite hard to kill and as opposed to both other forms can not be skinned, as their hides are far too mangy and burnt to be worth anything. Not surprisingly, they are immune to fire damage.
Fire geckos appear in Fallout 2 and Fallout: New Vegas.