Capranom

Capranoms were a type of goat that consumed massive amounts of food by first shrinking it down to size using their horns. Like goats, they were known for their stubbornness. Their ability to store energy meant they were frequently bred and reared to be used as electricity generators, themselves being like a living battery. When somehow they arrived in Norway, December, 1965, they began consuming the forests until they were rounded up by the Tenth Doctor using an artificial Christmas tree. They were handed over to a Capranom herder, who had travelled half a galaxy to find them, and were returned after they had consumed the unwanted trees after Christmas. (DWA: The Goats of Christmas Past)