From the Super Mario Wiki
Stretches are Boos attached to white platforms that appear in the game Super Mario Bros. 3. 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.
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".