Cat Grass is a type of cat treat found only in Nintendogs + Cats. The player can buy it from the BARC shop in the Toy Poodle version for $2, find it on walks, or when using your the pedometer feature. Like all other treats in the game, you can feed it to both your dog and cat, even though the name and description says it's supposed to be for cats.
3DS (NTSC): "This fresh oat grass satisfies a cat's natural craving for grass."
3DS (PAL): "Fresh oat grass to satisfy a cat's natural craving for grass."