Q'nidar are immense bats that set things on fire, and then absorb the heat as food.
Q'nidar look much like gigantic bats, with a hide made out of transparent, crystalline scales. Light and heat can be seen arcing around and through them, and when they fly, they leave streaks of radience in the air, like a vapor trail of light.
Q'nidar are attracted to large, flammable objects, such as ships, houses, or autumn fields. They light these objects on fire with their flaming breath weapon (a line of fire), and, after the flames have been burning for a few rounds, absorb the heat and light, extinguishing the flames. Q'nidar who are interrupted when feeding like this generally respond by feeding faster -- intensifying their fire, and dealing even more damage. Their main interest is on setting things on fire, and then absorbing the extra heat and light.
Perhaps odly, q'nidar are not invulnerable to the fire they create. They ignore most flames, but if one fire attack deals enough damage to overcome their remaining HP, they turn into a crystal form.
Q'nidar travel in flocks, feeding off the light and heat of each other. At the end of their lives, q'nidar migrate through the planes to a great source of fire (such as a star), and fall in, crystalizing as they tumble.
Q'nidar scales are useful lighting elements, glowing brightly after being heated just a little bit. Their fire-crystalized bodies are also useful for creating crystal balls,
2e AD&D Monstrous Compendium Spelljammer Appendix (1990); by Steven Schend.