Nightmare Treant
From Family Familiar. Didn't play nice with the other plants in the nursery. Elemental type. This should have been the best pet in the game man. A decent breed would be a start..
The 
Nightmare Treant is a Battle Pet rewarded by completing the Family Familiar meta-achievement during the Legion expansion, which requires beating any 12 of the 15 Master Tamers on the Broken Isles using teams of three pets from the same family across all 10 pet family achievement categories.
Guide awaiting review
Unlock Family Familiar
Complete all 10 pet family achievements by defeating any 12 of the 15 Master Tamers on the Broken Isles using a team of three pets from the same family — one run per family type, covering Aquatic, Critter, Elemental, Humanoid, Mechanical, Beast, Dragon, Flying, Magic, and Undead.
Tackle the Dragon Achievement
For the Dragon family achievement, fight Nightwatcher Merayl using a full team of Nexus Whelplings — use your highest-damage moves first, swap pets to reset cooldowns rather than waiting, and use your third move as a filler when the first two are on cooldown; any breed works, though P/S, P/P, and P/B have been used successfully.
Tips
For the Dragon family fights, alternative dragons with comparable high-damage abilities can substitute for Nexus Whelplings — choose whichever deals more damage as your filler move.
