There was only one because Edward is such a pansy, he couldn't have handled two. As for why it only fought him instead of destroying and/or stealing things... Notice it appeared while Edward was playing the harp? BushHags Sahagins are very critical of music, and they'll appear and try to kill you if they think you're a bad musician. Of course, since they're tone deaf, they can't differenciate bad music from good music...

How did the Dwarves cross the lava to build the town of Tomra to the south of the Underworld?