The Rubber Mushroom/Mushroom Toy (Japanese: きのこ,Kinoko) is an item in Nintendogs and Nintendogs + Cats that falls under the category of Sports in the Supplies. It is similar to the Bark Ball in the way that it bounces easily (due to it being rubber). It squeaks and bounces erratically. The Rubber Mushroom is yellow and red, while the Mushroom Toy is pink and white.
In Nintendogs, it bears a resemblance to the Mega Mushrooms found in the Super Mario series. The warning in the DS description is a hidden reference to the function of Mega Mushrooms: to make the user grow to great sizes.