As mentioned a while back, the Darkspear Rebellion event will be ending with the release of 5.4.
What does that mean for pet collectors? Gahz'rooki will no longer be purchasable and will turn into a rare drop. There will be a small chance of finding one "by killing the Kor’kron, and other associated mobs in Northern Barrens." You can read an official announcement here.
It's hard to say how "rare" this rare drop will be, and we'll have to wait to find out the approximate droprate. So if it seems like too much trouble or a hassle waiting for a drop, be sure to head over, do the small quest chain and start collecting the mats to purchase your own baby hydra. Or you can always check the AH, as this pet is also cageable. With that in mind, you could also purchase a few and save them to give to friends/family or to AH once 5.4 hits.
Either way, don't forget to collect Gahz'rooki. Although he already has 3 heads, I'm sure he'd enjoy your company. ;)