Firebats are a Thaumcraft mob found in the Nether. They resemble vanilla bats in size and sound, but are red with orange eyes. Unlike vanilla bats, though, firebats are hostile. They deal half a heart damage on all difficulties, but have a chance to set the player on fire on hard difficulty. Firebats have a chance of exploding near the player, dealing more damage. They drop 0-3 gunpowder on death, and have six hearts of health.
Also of note, these will spawn when the Aura's Flux reaches high levels. Keep an ear out for the bat squeaks when adding flux into the atmosphere. .
Heres a video that shows you firebats http://www.youtube.com/watch?v=O-jan5y6PoI