A Stretch is a variation of Boos that first appeared in Super Mario Bros. 3. They appear as long bars consisting of Boos that periodically appear.
After a long absence, Stretches appear in Mario & Luigi: Rivals Quest. In the overworld they will slighty follow the player, while some Boos appear and pop out of the Stretch and will turn into a normal Boo. The player must battle or quicky ignore them, or they will continuously make more Boos.
Like Pink Boos, pink variations of Stretches appear in the game. They are usually faster and more powerful.