Grog is an item sold by Innkeepers that gives you fire resistance when consumed.
Grog will not be Found in the creative inventory, as it is just a renamed version of the Potion of Fire Resistance from Vanilla Minecraft! It costs 64 emeralds, and is sold in the Inn, Dwarven Bazaar, and Market.
There are a few NPCs who will request Grog for certain rewards, like joining your team for instance.