Space-Boar

Space-Boars, were a race of giant carnivores whose excretions formed the Cassetia asteroids. Their primary food source was, according to the Eleventh Doctor, "rotting meat from the swamps of Malmatine 5". They were so incredibly large that they could travel in space without spacecraft. They travelled throughout the universe for over 20,000 years before their dung created the asteroids. The stench was so foul "that one whiff [could] make your hair fall out, and turn your eyes green" — so lethal that the atmosphere of a mine was comparatively healthy. (NSA: The Forgotten Army)