Corpulent Bonetusk

Covenant Feature: The Ember Court
Covenant: Venthyr

Covenant Feature: Queen's Conservatory
Covenant: Night Fae

Covenant Feature: Path of Ascension
Covenant: Kyrian

Covenant Feature: Abomination Factory
Covenant: Necrolord

From Warcraft Pets:

Members of each Covenant will have a different way of obtaining this battle pet, however each will need to apply upgrades to their hall to access certain Sanctum features.

Venthyr: After unlocking the Ember Court feature, members of this Covenant will "host parties" for different NPCs. At the end of the scenario a Tribute Chest will spawn and can be looted for rewards. The chests are specific to whichever NPCs attended the gathering.

Night Fae: After unlocking the Queen's Conservatory feature, members of this Covenant will be able to travel to a special garden and plant Wildseeds. These seeds produce nurtured souls, and each one will offer a chest with rewards. To upgrade Wildseed rewards, it must be connected to plants planted in at least 2 catalyst plots.

Kyrian: After unlocking the Path of Ascension feature, members of this Covenant will gain access to a vendor that sells this pet. Players must participate in the Path of Ascension to earn the achievement required to purchase this pet.

Necrolord: After unlocking the Abomination Factory feature, members of this Covenant can craft Atticus. To purchase the pets Atticus offers, players must be Honored with Stitchmasters.