There are many magical creatures living amongst us that go unseen by the non-wizarding communities. Most of them coexist quiet happily and without problems, but some of them a pose a threat to humankind. There are regulations on magical creatures because some of them are very dangerous. Dragons, for example, cannot be kept as pets.
They need properly trained handlers who can deal with their wild natures and who also can keep them concealed from the non-magical world. A more pocket-sized magical creature is a Niffler, a small long nosed fluffy creature who has a predilection for shiny objects. The most famous book on magical creatures is Fantastic Beasts and Where to Find them, written by well-known Magizoologist, Newt Scamander.