Stretch

Stretches are Boos attached to white platforms that originally appeared in the game Super Mario Bros. 3.

Super Mario Bros. 3
Stretches first appeared in Super Mario Bros. 3, where they are found exclusively in fortresses and are more of an obstacle than a true enemy. As Mario or Luigi attempts to pass, a Boo will rise from the top and bottom of the platform and attempt to hit him by moving left to right. Stretches are usually found in groups, making them a formidable foe. Stretches can only be defeated with a Starman or thrown hammers from the Hammer Suit.

Super Mario Maker
After many years of absence, Stretches reappear in Super Mario Maker. In the course editor, a Boo will automatically become a Stretch if it placed on top of a solid block. In this game, they can be placed on any surface, and will pop up more periodically, making them more of an enemy than in their original appearance from Super Mario Bros. 3. This new characteristic also makes them less predictable. Their appearance is also slightly different. New sprites for Stretches have been made for the Super Mario Bros., Super Mario World and New Super Mario Bros. U styles. However, they cannot accept mushrooms and giving them wings will make them faster. Attempting to give them a mushroom will turn the Stretch into a Big Boo but will also turn it back into a regular Boo. Big Boos cannot become Stretches in any way as of late.

A single Stretch appeared in the Nintendo Comics System short "In the Swim! Fun and Sun Fashions" in an advertisement for swimwear, which depicted it lifting weights on a beach. The Stretch, which only had one face, was shaped like a backwards "Z".