This is a handy little ne’er-do-well I came up with the other day to thwart my players. It worked wonderfully well and turned a hum-drum encounter into one that was surprising and dangerously fun.
Take a regular kobold with typical statistics. Add two heads, one on either side of the central head. They breathe fire and the kobold is immune to fire. It can do everything a normal kobold can do AS WELL AS breathe fire once per round from each head in a fifteen foot line.
They are incredibly weak due to being a kobold, but they can really pack a wallop! In the encounter in which I used them, the party wizard scoffed at a room full of twenty kobolds. With a flick of bat guano into the chamber he ignited a fireball that left all of the kobolds incinerated, except for the seven 3-headed kobolds, who in turn got to retort, and show said wizard just what that felt like. Evil grin.