The Magic Paintbrush is an item that first appeared in Super Mario Sunshine for the GameCube. In the game, it revealed that Professor E. Gadd made it, and that Bowser Jr, Bowser's son, stole it from him. Once he stole it, he covered the toxic ink that was in the paintbrush across the Isle Delfino.
Super Mario Sunshine
Super Mario Sluggers
Super Smash Bros. for 3DS/ Wii U