I'm looking for a piano piece from my music Lit class?

I'm trying to find a musical piece from my music lit class years ago. I remember it started with triplets in the left hand to denote people/horses riding along and featured a monster like creature. And, if I'm not getting this piece mixed up with something else then the story went on to have this monster go after a child (yes, messed up, but that's what I remember of it). The triplets are really the distinctive part of this piece though, with the melody in a minor key, I think it was possibly written by Wagner but I'm not positive; it was an influential piece though, or else it would not have been studied in class. Does anyone out there have any clue what I'm talking about?