It looks similar to a normal Super Mushroom, but skinny similar to the way Mario turns skinny. The Weird Mushroom has a 1 in 10 chance of spawning wherever a Super Mushroom is placed. However, it can also be unlocked by clearing all four of the Nintendo World Championships courses. Upon unlocking the Weird Mushroom, the player can shake the normal mushroom and get a Weird Mushroom. The Weird Mushroom is only available in the Super Mario Bros. theme so if the player changes the game style, every Weird Mushroom object will turn back into a normal Super Mushroom.
In the E3 2014 version of the game, the Weird Mushroom was called a Luigi Mushroom and just stretched Mario's sprite. It was said to be a result of a glitch.