Young Mutant Warturtle
From Lhara. Mmm... Pizza! Aquatic type. Got mine just now because someone else had put down the turtle item but everyone was too busy fighting a rare to notice and grab the tames.
The Young Mutant Warturtle is a wild battle pet found in the Underbelly during Legion, obtained by purchasing and using a summoning item from a vendor near the arena area.
Guide awaiting review
Buy summoning item first
Head to the Underbelly arena area and talk to Laura Malley to purchase the Young Mutant Warturtles item, then right-click it in your inventory to summon the turtles in front of her.
Hire a bodyguard first
Before summoning, make sure you have an active bodyguard, as the Underbelly is a PVP zone — other players can hit you or AoE you to forcibly end the pet battle, causing you to lose the turtle and the Eyes you spent.
Start the pet battle
Once the turtles appear, hovering over them will not show the usual pet battle cursor — simply right-click on a turtle and the battle will begin; they typically pair with two Frog companions.
Upgrade rarity after capture
If you capture the Young Mutant Warturtle at a lower rarity, you can purchase a rarity upgrade stone for 15 Pet Charms from your garrison vendor.
Check Darkmoon Island vendor
Lhara at 48.2, 69.4 on Darkmoon Island is also listed as a source for this pet, so check there as an alternative option.
Tips
The Young Mutant Warturtle displays a small bandanna above its eyes and crossed swords on its back when summoned, though it does not have unique click sounds.
Sometimes another player may summon the turtles and leave without battling them — if you spot the paws icon on the ground, right-click the turtle to claim the battle for yourself at no cost to you.