The Rubber Mushroom (Mushroom Toy in 3DS) is a toy that can be found on walks. The Rubber Mushroom is yellow and red, while the Mushroom Toy is pink and white. It bears a resemblance to the Mega Mushrooms found in the Super Mario series in Nintendogs. The Rubber Mushroom is similar to the Bark Ball in the way that it bounces easily (due to it being rubber). It squeaks and bounces erratically. The warning in the DS description is a hidden reference to the function of Mega Mushrooms: to make the user grow to great sizes.
DS: "When thrown or chewed, this little toy makes a lot of noise (do not eat)."
3DS: "This toy makes a lot of noise when it's thrown or chewed on."