Creatures from Below

Creatures that originate from the Underdark, but have found a way into the normal world. These creatures are seen by most as 'really strange creatures, whereof it's origins are completely unkown'. Many of these creatures are completely different than anything else in the normal world. This gives them really wierd traits and abilities, that can't be found on anything else. To find one of these creatures is extremely rare. Most of these creatures are being researched all the time by scientists, trying to find out where it came from. None have had succes with this, because the answer: "It came from a world bellow this one" is not good enough. This is because most scientists don't believe in the Underdark, because it has never been proven that it exists. But guess what? It does exist.