From the Super Mario Wiki
Super Mario Bros. 3
Stretches first appeared in Super Mario Bros. 3 and its respective remakes. They are found exclusively in fortresses and are more of an obstacle than a true enemy. As Mario or Luigi attempts to pass, it 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 hammers thrown by Hammer Mario.
Super Mario Maker
After many years of absence, Stretches reappear in Super Mario Maker (only referred to as "Boos"). 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, and they are not connected to a white platform anymore. 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 rather than bigger Stretches.
Super Mario Run
Stretches also appear in the game Super Mario Run. They are usually found in haunted house courses and act like they did in Super Mario Maker. They have the same appearance as the Stretches in the New Super Mario Bros. U style from Super Mario Maker.
Nintendo Comics System
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 "Z".
Official profiles and statistics
Names in other languages