Whoops, spoke too soon.  What looked like a giant house cat is actually a Displacer Beast.  These horrible creatures have been around since the first edition of D&D, but in case you're not familiar with them you should know that they are never where they seem to be.  Being displaced from reality makes them really hard to hit.  It's not as bad as being invisible but it can be just an annoying.  The group is going to have a harder time with this encounter then we thought.