UPDATE: Blizzard recently stated the following about Tenris Mirkblood, the new boss that drops the pet batling:
He's around for approximately 10 days. Because he's rare, he drops a bat for everyone.
So in case anyone was wondering how long they had to get their pet, it sounds like the boss will only be available through Monday, November 3rd.
NEW PET! The Scourge Invasion brings us yet another new pet! This one is the Vampiric Batling. It looks as though it may be limited to high level characters, as it's dropped from Tenris Mirkblood, a new raid boss in Karazhan. A group of ten or so is required to bring down Tenris, but the good news is that every single raid member will be able to loot a batling! However, this boss will only be around during the invasion, and this is a one-time per server event. So what are you waiting for!? Go fight some Scourge and claim your bat!