Thanks to all the Magic 2015 promotional material, everyone now knows what happened to Garruk during the events of Innistrad block: Liliana used the Chain Veil on him during their fight and partially cut off his connection to green mana, suffusing him with black mana instead and turning him into a Jekyll-and-Hyde-like twisted monster of a man. The latest Uncharted Realms story on magicthegathering.com, "Beast" by Tom LaPille, shows us part of what's happened in the interim - and also lets us know that Garruk is now summoning creatures who are cursed like him or undead. This extends to the ones he summoned previously... like the infamously powerful Thragtusk. And the new art shown in the article indicates that the corrupted 'Tusk will be in the upcoming core set.
The relevant passage in the story:
A beast as tall as me and as wide as it is tall approaches me and sniffs my still form. It smells mostly right, but there's decay underneath, the same as what's always coming off of me now. Under its fur, its skin is shot through with black veins, just like mine.
I summoned it during the fight with the witch.
The art connection is clear: A shaggy, bovine-shaped monster with two curved horns and two curved tusks. The art department wouldn't let a coincidental resemblance to one of the most famous and powerful recent Magic cards pass unaltered, so we're looking at an intentional callback.
Any ideas as to what a black Thragtusk could look like in card form? Here's my take:
Creature - Zombie Beast (Rare)
When Dregtusk enters the battlefield, destroy target creature with power 3 or less.
When Dregtusk leaves the battlefield, each player loses 5 life.