Grace is a giant green mutated boar found under Bob's Boars, or at 700N;300W in Navezgane. The very same farm also contain samples of Super Corn, therefore it is a possibility that Grace is mutated due to its consumption of the mutated plant.
Grace has a decent health pool, comparable to a radiated zombie, but it will not attack the player unless attacked first. In fact, Grace is the only neutral zombie found in the game. Interestingly, harvesting Grace yields large amounts of fat, raw meat, leather, and more large bones than usual, suggesting Grace may not even be a zombie, but merely a super pig.
Grace is found in a cave concealed under its cage in the backyard, with its name printed on it. Killing Grace is essential to completing clear zombie mission.
Sometimes a bear or nothing at all would spawn in place of it.