From J Gilbert's website: 
  
Q:  Is there a story behind the squirrel and grapes motif? 
  
A: I don't know of any textual reference, but squirrel and grapes is a very common kodogu motif.  People in the US often mistake the squirrel for a fox. I can understand that, since Aesop’s fable of the fox and grapes is more familiar in the West. 
  
The squirrel and grapes motif probably entered Japan in the Nara period by way of Tang China via the silk road.  It may have originated in Greece.  It’s a popular decorative motif, but has additional connotations for sword fittings.  The squirrel and grapes motif is a visual pun for perfection in military achievements.  The Japanese word for grape is budo, and that is of course a homonym for "way of the martial arts."  The word for squirrel, rizo, is also a homonym with the kanji that mean something like "setting high ideals."  Once you figure that out, it's pretty easy to understand why it was such a popular "picture."  It was very common in late Muromachi - Momoyama Heianjo zogan work and remained popular at various times through the Edo period.