Gahz'rooki
Source: Vendor. The trolls have a long and storied history of summoning hydras. While he might not be the biggest or strongest hydra, Gahz'rooki will bite your fin... Aquatic type. He's a reference to the Godzilla cartoon from 1978 in which Godzilla has a nephew named Godzooky who is the comic relief character.
Gahz'rooki is a hydra battle pet from the Classic expansion, obtained by spending Radical Mojo — a currency dropped by Kor'kron and Hellscream mobs in the Northern Barrens — or as a rare drop from those same mobs in the form of 
Gahz'rooki's Summoning Stone.
Guide awaiting review
Farm Kor'kron Mobs
Head to the Northern Barrens and defeat Kor'kron Marauder and Hellscream Laborer mobs at 38, 20. These enemies drop both 
Radical Mojo (used as vendor currency) and Gahz'rooki's Summoning Stone as a rare direct drop.
Redeem Radical Mojo
Once you have enough Radical Mojo, spend it at the associated vendor to purchase Gahz'rooki. Note that Gahz'rooki's Summoning Stone is Bind on Pickup when it drops directly, but once you learn the pet you can cage and trade it.
Tips
Gahz'rooki exists in only one breed (Power/Speed) with final stats of 1400 health, 297 power, and 281 speed, making him a reasonable choice in PvP pet battles for damage output and turn-order advantage.
His moveset blends beast and aquatic attacks with a single elemental move — consider keeping at least one aquatic ability on your bar unless you are specifically countering critter-type enemies.
Tail Slap deals roughly 20% more damage than Bite but hits less frequently; both Swallow You Whole and Devour are most effective against low-health targets, so slotting at least one of them is worthwhile.
