The Lava Golem is rumored to lurk beneath lava and is rumored to be the cause of missing miners.  It feeds off of other monsters and can withstand any amount of heat.  If you cover up a lava patch it only makes the Lava Golem more angry.  Also, beware of the face on its back.  The face is rumored to lure miners.  The only way to kill one of these beast is by the power by water.