Hachiemon is a 'Tanuki' and is also Miroku's partner. Miroku often calls him Hachi as a nickname. In the days before Miroku joined Inuyasha Hachi and Miroku would scheme up elaborate scams were Hachi would tranform into a youkai and Miroku would pretend to purify it in order to get food, shelter and money.
Hachi, as he is called for short in the series is a Raccoon-Dog or Tanuki, which is actually neither a Raccoon or a Dog. Throughout the series several jokes are made about him being a Raccoon-Dog.
It should also be noted that he seems to have the ability to turn into a giant yellow blimp creature and fly over vast distances while carrying Inuyasha and his friends on his back.