I was kind of wondering why I didn't see this yet, so I took the opportunity to make it my self.
For all of you poptart cat loving people out there, now you can adopt your very own Nyan Cat.
It's a cat, but it's also a poptart. Because why not?  
  
  
  
To install, just go to your Stardew Valley content/animals folder:  
  
Steam\steamapps\common\Stardew Valley\content\animals  
  
And replace the cat.xnb with the downloaded one provided.
BACK UP THE ORIGINAL CAT by renaming the old one to cat_OLD.xnb or something similar