The Slitheen are a family of aliens from the planet Raxacoricofallapatorius. They go around the galaxy causing trouble and their main aim in life is to make as much money as possible regardless of whom they harm in the process. In The Lost Boy, they planned to bring the moon crashing down. To defeat these green monsters, you need to throw vinegar-based products on them, as they are made of calcium and the result is a huge, gooey explosion.