The Bandix are small creatures not related to any creature in Arda. They kill/steal from people in the Region of Eriador. They are small creatures who are evil.
The Bandix have unknown origins, some say they were created by Melkor or Sauron. While Others say Eru made them as a mistake. They were known to protect hoards of gold and other things. One adventurer even reported he found a Silmaril in one of the Bandix hoards, but that simply cannot be true.