The Firebat is a hostile mob added in Thaumcraft 3. It is a red bat with yellow eyes. It makes fire particles.
It spawns naturally in the Nether with a rarity in between a Zombie Pigman and a Ghast. It could also spawn in the normal world when the Flux level is high.
They attack the player by making contact and setting the player on fire, sometimes killing itself in the process.
The rod of nine hells Wand can spawn firebats. It has nine loading bars above your health, and every bar loaded allows you to spawn a bat if a mob is near by.
It drops 1-2 gunpowder when killed. It can fly like a bat and has a little more health than a normal bat.
They can be toggled off in the config file.
Here's a video that shows you firebats: