Obtaining this pet naturally follows crafting a Wirt's Leg. Instead of the usual Forgotten Soul, salvaging a Wirt's Leg yields a Map of the Stars, reading the sequence '2-1-3'. Head back to Adria's Hut in the event area, and note the cow corpses outside. Click them in the described sequence, or more specifically: middle, left, right. This triggers the Abandoned Farmstead entrance to the right; a small area whose only notable feature is the "Wirt's Stash" chest in the middle. It contains the Royal Calf pet and some gold.